building and unit start class, many do something like resource.. todo
	new file:   LibreSTR/Building.cpp
	new file:   LibreSTR/Building.h
	modified:   LibreSTR/CMakeLists.txt
	modified:   LibreSTR/GameMap.cpp
	modified:   LibreSTR/GameMap.h
	modified:   LibreSTR/GameState.cpp
	modified:   LibreSTR/MainMenu.cpp
	new file:   LibreSTR/Unit.cpp
	new file:   LibreSTR/Unit.h
	new file:   assets/sprites/building.png
	new file:   assets/sprites/building.spr