This commit change the screen edge function calls to be called in the
new class ScreenEdge. The old methods are still in Workspace and will
be removed in a further commit.
This initial commit introduces a two new files screenedge.h and screenedge.cpp which cover a new
class ScreenEdge. The code for screen edge handling was copied from Workspace to this class.
Workspace had to be extended by a getter for movingWindow. CMakeList was updated to build the
new class.