[Please Test] Refactoring of System Window Behavior

Hi, there!

I did another refactoring of SystemWindow. It primarily addresses the changes around fall 2015, where we got the feature that background windows do not raise with any click while still being interactive and not locked.

Here is the refactoring: all-windows-focus-refactoring.cs

The preference to play around with are:
- windows raise on click
- windows accept input on first click
- background windows lock title bar
- focus follows mouse

All present in the "windows" category in the preference browser.

Best,
Marcel