identify maximized window screen differently

--there are cases that the window geometry isnt consistent
for maximized windows. That was breaking the previous
implementation of dynamic background because of:
https://bugs.kde.org/show_bug.cgi?id=397700
the new implementation is using only the window center in
order to identify the screen in which a window is
present

BUG: 397344
FIXED-IN: 0.8.1