gtk3: colorpicker exception fix for some setups

Fix attempt. Not sure why this is a problem, I didn't get an exception.
https://gna.org/bugs/?20791
stategroup: fix GTK3 exception in activate()

Fix exception when activating GUI States (e.g. Color History) which have
been bound to a mouse button in preferences. The class hierarchy is
different for GDK events now; we can't use it for tests.

https://gna.org/bugs/?20823

Document method too.
remove gtk input dialog

no longer supported in gtk3, useful only to workaround gtk bugs
Merge MyPaint master into MyPaint-xsheet