Add folder to workspace... dialog should support multiselection

[colin-grant-work]

The dialog that opens when you select the 'Add folder to workspace...' menu item should support multi-selection to facilitate adding multiple folders to a workspace simultaneously. The corresponding VSCode dialog does support multiple selection.

Feature Description:

The command is registered here:

theia/packages/workspace/src/browser/workspace-commands.ts

Lines 306 to 332 in 0ebdec5

	registry.registerCommand(WorkspaceCommands.ADD_FOLDER, {
	isEnabled: () => this.workspaceService.isMultiRootWorkspaceEnabled,
	isVisible: () => this.workspaceService.isMultiRootWorkspaceEnabled,
	execute: async () => {
	const uri = await this.fileDialogService.showOpenDialog({
	title: WorkspaceCommands.ADD_FOLDER.label!,
	canSelectFiles: false,
	canSelectFolders: true
	});
	if (!uri) {
	return;
	}
	const workspaceSavedBeforeAdding = this.workspaceService.saved;
	await this.addFolderToWorkspace(uri);
	if (!workspaceSavedBeforeAdding) {
	const saveCommand = registry.getCommand(WorkspaceCommands.SAVE_WORKSPACE_AS.id);
	if (saveCommand && await new ConfirmDialog({
	title: 'Folder added to Workspace',
	msg: 'A workspace with multiple roots was created. Do you want to save your workspace configuration as a file?',
	ok: 'Yes',
	cancel: 'No'
	}).open()) {
	registry.executeCommand(saveCommand.id);
	}
	}
	}
	});

I tried a naive implementation just setting canSelectMany to true and setting up a loop over the selected URI's, but that didn't work immediately, so some additional modification of the workspace adding code is likely necessary.