pdWindowManager: myriad improvements and clean-up

Now that I'm more familiar with the inner workings of Windows' handling
of windows, I'm better able to use features like layered windows to
improve performance, while also clearing out unnecessary code left over
from when floating image windows were supported.  This not only makes
PD's window manager code cleaner, but it also has a non-trivial impact
on performance - particularly startup performance, where the window
manager is one of the biggest time sinks.

I'd like to continue to trim down this class, but because changes have
sweeping repercussions across OS version, theme settings, and many other
factors outside my control, any modifications need to be made gradually,
and carefully tested on as many PCs as possible, to make sure I haven't
accidentally broken behavior under atypical OS/theme arrangements.
(Note that custom window management features like Aero Snap under
unthemed Win 7/Vista are the real troublemakers, because the behavior is
implemented *totally differently* than it is under Aero - so test
carefully!)