New Window Manager class

With the move away from MDI, PhotoDemon will now have to manage its own
Windows.  This helper class is designed to aid with that.

At present, its primary responsibilities involve subclassing toolbar
windows, and tracking their position relative to the main form.
Similarly, it also subclasses the main window, and moves all child
windows when necessary to maintain relative layout.

I expect this class to grow significantly as we migrate more and more
functions away from MDI...
Convert primary and selection toolboxes to floating windows

Lots of bug-testing and small UI stuff still to fix/add, but the program
compiles and the new floating dialogs are fully usable.