update xbm handling
replace icon button with bitmap button

The bitmap button is more flexible since it can be styled via qt style
sheets.
replace icon buttons with bitmap buttons
replace icon buttons with bitmap buttons
replace tab close button with bitmap button
expose the qt stylesheet directly for the DockArea

The Qt stylesheet provides a lot of functionality which would be
difficult and tedious to wrap. It's far simpler to allow the developer
to define the qt stylesheet directly.
fix an item comparison bug
get rid of testing file