app: remove support for legacy conf, tidy saving

Remove support for loading the old settings.conf file.

Tidy config saving.
tdw+others: remove some pre-GTK3 code (NFC)
app: move at-start callback to its own method

Make the constructor a little less messy.
kbm: cancel key capture on window destroy

Avoids a memory leak with dynamic subwindows.
linemode imports: tidy and defer

Prevents some more import circularity.
app imports: tidy and defer
brushselectionwindow: resize fixes

Remove another self-resize, replace with GTK3-style sizing.
toolbar: remove MainMenuButton
Merge branch 'mypaint-head' up to 7987bfa and resolve second conflict

Conflicts:
	gui/toolbar.py