We need to update all button rectangles when the layout changed.
We also need to clear the taskbar button rectangle when we
remove a modal window button, so that WindowServer uses the
parent's taskbar button rectangle for the minimize animation.
With this, System Monitor has "System Monitor" instead of
"SystemMonitor" as tooltip, matching the app's title bar
and menu bar title.
Same for File Manager and Text Editor.
If a window in the taskbar has progress, we'll now draw that progress
in the form of a progress bar behind the window title on the taskbar
button for the window.
Previously we were wasting the bottom pixel row on darkness. Use the
base button color all the way to the bottom row and offset the top
highlight by one pixel instead.
