Since the functionality of screen edge handling was moved to its own class,
the screen edge windows are now provided by the getter method screenEdgeWindows().
The new getter method screenEdgeWindows() returns a reference to a QVector,
which includes the screen edge windows.
The Workspace method propagateClients() implemented in layers.cpp needs these
screen edge windows to restack the windows. The code was also modified to use
QVector instead of a Window array.