Windows that are marked as modified will now have another (themable)
close button. This gives an additional visual clue that some action
will be required by the user before the window gets closed.
The default window-close-modified icon is an "X" with "..." underneath,
building on the established use of "..." in menus to signify that
additional user input will be required before an action is completed.
This implements simple window shadows around most windows, including
tooltips. Because this method uses a bitmap for the shadow bits,
it is limited to rectangular window frames. For non-rectangular
window frames we'll need to implement a more sophisticated algorithm.
