windowing: minor refactor for dialogs, +other NFCs

Base ChooserDialog on the ordinary Dialog class for a little more
commonality and code-sharing.

Use a better system for invoking the real hide_on_delete()

Various comment reorderings and fixes.

Break out long commasep'd imports to separate lines.
Merge branch 'mypaint-head'

Resolve another conflict (third at this point), commit was mostly just formatting adjustments

Conflicts:
	gui/drawwindow.py