Fix status bar message size when image window is tiny
Fix many small layout issues and bugs

A lot of bug reports about various window and UI elements have been
piling up due to me having reduced PD time over the holidays.  This
commit aims to clear out most of the simple fixes required.
Remove all vestiges of old progress bar class

We only use official system progress bars now
Convert plugin downloader to system progress bar

Also, fix some lingering layout issues