[Theia][Plugin]Rework Task extension to Task plugin

[vparfonov]

The existing https://github.com/eclipse/che-theia-task-plugin is developed based on the existing extensibility model in Theia IDE. But as far as new plugin model is on it's way and will be used in Che as the main one - there is need to rework Task extension to Task plugin.

P.S.: add subtask here if need

Missing Theia Plugin APIs to implement:

• tasks.registerTaskProvider [plug-in][tasks] Register a task provider eclipse-theia/theia#3503
• tasks.onDidStartTask [plug-in][tasks] Add tasks.onDidStartTask API eclipse-theia/theia#3505
• che.registerVariableProvider [Theia][Plugin] Add VariableProvider API #11953
• che.registerTaskRunner [Theia][Plugin] Add TaskRunner API #11954
• Fix 'onDidSelectItem' behavior for Quick Pick widget [plug-in] Fix 'onDidSelectItem' behavior for Quick Pick widget eclipse-theia/theia#4046
• Ability to open URL in the Preview Tab Add ability to pass argument for 'mini-browser.openUrl' command eclipse-theia/theia#4243
• Ability to get current selection in the explorer view [plug-in] Ability to get current selection in the explorer view eclipse-theia/theia#4043
• Theia Terminal plugin Theia Terminal plugin: add missing API and implement the Plugin #11442
• Ability to specify container for webview panel Ability to specify container for webview panel  eclipse-theia/theia#4257
• Ability to check whether the webview panel is active and visible [plug-in] Ability to check whether the webview panel is active and visible eclipse-theia/theia#4342
• Implement 'WebviewPanel.reveal() [plug-in] Implement 'WebviewPanel.reveal()' eclipse-theia/theia#4339
• Add 'tasks.taskExecutions' API [plug-in] [tasks] Add 'tasks.taskExecutions' API eclipse-theia/theia#4372
• Improve the way of storage runtime info for Che tasks https://github.com/eclipse/che-theia/issues/78

To implement Task plugin features progressively we can implement it in the following order:

• once the tasks.registerTaskProvider API is done
  • implement providing the Che workspace commands as Theia tasks
• once the che.registerTaskRunner API is done
  • implement CheTaskRunner for running the tasks via machine-exec server
• once the che.registerVariableProvider API is done
  • implement resolving the variables in a Che task
  • add VariableProvider for resolving server's address by name, e.g.: ${server.tomcat}
• once the tasks.onDidStartTask API and Terminal API is done
  • make it possible to connect to machine-exec container to show a task's output
  • implement Preview URL functionality(info message with ability to open preview URL in the preview widget of Thea project or in a new tab of browser)
• once ability to specify container for webview panel is done
  • implement widget for displaying list of preview URLs (server name ---> Preview button ---> Go to button) and ability to display it from status bar item
• once the 'tasks.taskExecutions' API is done
  • add ability to display preview URL for running tasks
• once ability to get current selection in the explorer view is done
  • implement project path variable resolver, e.g.: ${current.project.path}
• once Theia Terminal plugin is done
  • switch task's output from output channel to terminal widget
• once Webview API is improved
  • improve way of updating previews widget

[azatsarynnyy]

After analyzing the che-theia-task-extension sources I see the following steps are required to start reworking the extension as a plug-in:

• add taskDefinitions contribution point
  https://code.visualstudio.com/docs/extensionAPI/extension-points#_contributestaskdefinitions

• extend Theia Plug-in API with Task API
  https://code.visualstudio.com/docs/extensionAPI/vscode-api#_tasks
  note, it may require to adapt the existing Theia Task Extension API to better fit with Plug-in API

• provide a possibility to register a Task Runner
  https://github.com/theia-ide/theia/blob/master/packages/task/src/node/task-runner.ts#L34
  note, there's no correspondent API in VSCode

• make it possible for plug-in to contribute to Theia's preferences

• implement WebView API for Preview URL functionality
  https://code.visualstudio.com/docs/extensionAPI/vscode-api#Webview

• add Plug-in API for registering the Variables (former macros)
  https://github.com/theia-ide/theia/blob/master/packages/variable-resolver/src/browser/variable.ts
  note, there's no correspondent API in VSCode

Terminal

In order to show a task output I guess the che-theia-terminal-extension need to be reworked into a plug-in and it should provide some API to connect to machine-exec server.

[azatsarynnyy]

Useful links that might be helpful:

• Theia Task Extension https://github.com/theia-ide/theia/tree/master/packages/task
• VSCode tasks user's guide https://code.visualstudio.com/docs/editor/tasks
• VSCode Task API guide https://code.visualstudio.com/docs/extensions/example-tasks
• VSCode Task API reference https://code.visualstudio.com/docs/extensionAPI/vscode-api#_tasks
• VSCode taskDefinitions contribution point https://code.visualstudio.com/docs/extensionAPI/extension-points#_contributestaskdefinitions

[azatsarynnyy]

I've updated the description with the subtasks.

[slemeur]

How are we doing on this issue and what's remaining? is the task list in the issue description up to date?