More various minor interface fixes and improvements

- Loading multiple images at once is working again
- SDI image tabstrip size is now saved/restored
- when multiple images are loaded, and one is closed, focus is set to
the next image in line (so the SDI image tabstrip now works like a
proper tab strip)
- image window activation calls are now tracked via debug printing,
which helped me eliminate some inefficiencies in the way windows are
refreshed
- unsaved changes dialog is now owned by whichever window called it,
preventing it from being hidden under document windows
- SDI image tabstrip is less garish
- image window function ActivateWorkaround is now used in multiple
places instead of unpredictable SetFocus calls