Add gfx::Region class

This class will be used to replace JRegion in ui lib.
Replace ui::jregion with gfx::Region class

This is an old TODO item, and it's the first step to finally
get rid of JRect and struct jrect.
Merge branch 'dev' of git@github.com:dacap/aseprite.git into dev

Conflicts:
	src/ui/popup_window.cpp
	src/ui/tooltips.cpp
	src/ui/tooltips.h
	src/ui/window.cpp