When a window overlaps another window it would be nice if it could turn semi
transparent so that the window behind it is viewable. I would be more than happy
to work on this myself but have little time to bury myself in xfwm4 code. If
someone could give me pointers on a way to tell if windows are overlapping
inside xfwm4, or even outside of it somehow I will start to develop this feature. 

Reproducible: Always
Steps to Reproduce:
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