Log: turn log into an interface.
Core: add a Log property.
Core: add an exemple of how to set up the requested callbacks.
Core: try to implement the interface setting mechanism (not working yet).
Makefile: produce a internal header for retro.
Core: set the log callbacks from the log property.
Core: put the global Core into its own file with accessors.
Core: put interfaces setters in their own file.
Rumble: turn Rumble into an interface.
Sensor: turn Sensor into an interface.
Camera: turn Camera into an interface.
Performance: turn Performance into an interface.
Performance: turn Performance into an interface.
Location: turn Location into an interface.
Merge branch 'interfaces' of github.com:Kekun/retro-gobject into interfaces

Conflicts:
	retro/Core.vala
	retro/retro-core-interfaces.c
	retro/retro-core-interfaces.h