workspace: simplify fullscreen & maximized starts

If the window needs to be fullscreened or maximized at startup, just ask
for it up front rather than deferring the request. Seems to work here for
me in Xfce, anyway.
freehand: fix missing import
linemode: fix typo-based AttributeError
brush/color history: refactor

Factor out colour and brush history code to a separate module, and update
the coding style a bit. GTK3 now for this part, and make the click
notification more consistent between the widget classes.
history: very basic brush & color hist panel

Add a very basic brush and colour history panel. I may split this up. Icon
is just a placeholder for now.
quick brush chooser: update sizing code

The dropdown can no longer rely on the history widget being big enough to
bump it out to 4 brushes wide, so set a min size the GTK3 way.
Merge latest changes from MyPaint HEAD

Conflicts:
	gui/drawwindow.py