fixes #140
Initialized with nullptr to test against that when trying to access "ghost_stone"
fixes #140 Now correct creating and deleting of ghost_stone in dependence of "scene.clear()"
Merge remote-tracking branch 'remotes/origin/master' into #140_Memory_Access_violation

Conflicts:
	Go_GUI/VirtualView.cpp
Merge pull request #142 from augmented-go/#140_Memory_Access_violation

#140 Memory Access violation