[[ IconGravity ]] Initial implementation of button iconGravity property - persistence still to do.
Merge branch 'develop' into feature-icon_gravity

Conflicts:
	engine/src/control.cpp
	engine/src/graphicscontext.cpp
	engine/src/parsedef.h
[[ IconGravity ]] Tweak to signature of drawnodata.
[[ IconGravity ]] The iconGravity now persists.
Merge branch 'feature-icon_gravity' into develop