Introduce Workspace Folder Pick API

[ashumilova]

The new plugin model should contain the API for picking workspace folder. The task includes following items to be done:

• define the Workspace Folder Pick API
• document the API
• develop the defined API
• test it on the sample plugin

[vitaliy-guliy]

Code to test

    const pickWorkspaceFolder: theia.Command = {
        id: 'plugin-api-pick-workspace-folder',
        label: 'Pick Workspace Folder'
    };

    disposables.push(
        theia.commands.registerCommand(pickWorkspaceFolder, (...args: any[]) => {
            theia.window.showWorkspaceFolderPick().then(value => {
                if (value) {
                    theia.window.showInformationMessage('Workspace ' + value.name);
                    theia.window.showInformationMessage('Path ' + value.uri.path.toString());
                } else {
                    theia.window.showInformationMessage('Canceled');
                }
            });
        })
    );