Refactor|UI|Task Bar: Custom widgets as submenus; improved task bar behavior

Added a SubwidgetItem class that allows one to create a custom
popup widget as the submenu of a menu item. MenuWidget now knows
how to handle these items.

Refactored task bar to use SubwidgetItem rather than its own
solution for creating the various configuration dialogs.

When the console command line gets focus, task bar closes any open
menus.