Replace MenuBar with PopupPanel on ComponentCell

Now, on any click in the ComponentCell, the menu pops up below the
ComponentCell.

Note that I commented out the old code. It'll need to be cleaned up
later.
Set styles on ComponentSelectionMenu
Activate ComponentSelectionMenu

I'm using a ClickHandler on each menu item that calls
ComponentSelectionCommand. Later, refactor ComponentSelectionCommand
and put its action into the ClickHandler.
Convert ComponentActionMenu into a PopupPanel
Activate ComponentActionMenu