Simplify QuickEditor

In this PR I did some refactoring:

1. Replaced QRegion mScreenRegion with QRect mScreensRect to avoid extra conversion (QRegion is never directly used needed) and called preparePaint() earlier to reuse mScreensRect for getting pixmap size (this .
2. Moved some complicated stuff from constructor to separate functions.
3. Changed header formatting in accordance with the used style in new files.
4. Moved reusable lambdas to static functions.
5. Added support for screen geometry with negative coordinates. This was is not a problem on X11, but it is possible on Wayland and Windows. Will send in a separate PR.
6. Passed WindowFlags to the QWidget constructor directly.

I tested changes on X11 and Wayland.