Simplify QuickEditor

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.
2. Moved some complicated stuff from constructor to separate functions.
3. Changed header formatting in accordance with the used style in new files.
4. Passed WindowFlags to the QWidget constructor directly.