Reworked the widow class hierarchy a bit.

	renamed:    gui/sdl2/Window.cpp -> gui/SDL2Window.cpp
	renamed:    gui/Window.h -> gui/SDL2Window.h
	renamed:    gui/WindowBase.cpp -> gui/Window.cpp
	renamed:    gui/WindowBase.h -> gui/Window.h
Added TextureAtlas class and dynamic subtexture allocation.

Needs more testing.
Touched a couple of textures.
Added relative mouse movements for knobs.
Added relative mouse position switching and polling methods to Mouse class.
Fixed gui/CMakeLists.txt after renaming Window classes.
Added a couple of usefull comments and some methods to classes in geometry.h
A pretty knob background.
Fixed a lot of files after Window class renaming.
Using Window_t typedef.
Added << operators for geometry primitives.