Taskpane widget's task list menu should contain all taskpane widgets

[zjames]

The taskpane widget contains a menu displaying a list of available taskpane widgets. Currently, the widgets included in this menu must be manually added to a named menu container in the application definition. If additional taskpane widgets are included in the layout, they are not displayed automatically.

The taskpane widget should be enhanced to automatically enumerate widgets that target the task pane and add them to the menu. It should also be possible for other containers (i.e. toolbars and menus) to include the collection.