multislection for ground unit
	modified:   LibreSTR/CMakeLists.txt
	modified:   LibreSTR/GameState.cpp
	new file:   LibreSTR/gui/MultiSelectionPanel.cpp
	new file:   LibreSTR/gui/MultiSelectionPanel.h
	modified:   LibreSTR/units/GroundUnitIdentity.cpp
	modified:   LibreSTR/units/GroundUnitIdentity.h
	modified:   assets/worker.res
	new file:   assets/worker_icon.png