Support multiselection in 'Add Folder to Workspace...' dialog

[colin-grant-work]

Signed-off-by: Colin Grant colin.grant@ericsson.com

What it does

Fixes #9665 by allowing multiple selection in the workspace dialog and ensuring that the public methods 'addRoot' and 'removeRoots' (and new method addRoots) don't return before the ._roots field of the WorkspaceService has been updated. Previously, changes to the workspace would emit an event that would trigger an updateWorkspace cycle, but since that method is also asynchronous, it was guaranteed not to have finished its work in the tick after addRoot and removeRoots resolved, meaning that even awaiting those methods didn't guarantee that the WorkspaceService was ready for another call.

How to test

1. Open a workspace.
2. Use the 'Add folder to workspace' command (available in the ... context menu in the explorer).
3. Select several folders.
4. Observe that your workspace updates correctly to include all of the folders added.

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

Looks good to me. I can confirm that the changes allow to select multiple folders in the browser and Electron (Ubuntu) environment.

[vince-fugnitto]

I confirmed that the behavior works as intended:

• verified without a *.theia-workspace file present and adding multiple roots
• verified with a previous *.theia-workspace file present and adding multiple roots
• adding a single workspace root works as before

[vince-fugnitto]

The changes work for me 👍

[vince-fugnitto]

I believe it can be simplified to:

async addRoot(...uris: URI[]): Promise<void> {
    await this.spliceRoots(this._roots.length, 0, ...uris);
}

Calls like the following are allowed:

await this.workspaceService.addRoot(...workspaceFolders);
await this.workspaceService.addRoot(uri);

But I won't nitpick :)