Cleanup QuickEditor code

* Remove reference for trivially-copyable types.
* Add reference for non-trivially-copyable types.
* Remove extra variables.
* Use QTransform::fromScale (it faster: https://doc.qt.io/qt-5/qtransform.html#fromScale).
* event->pos() assigned to mMousePos, so just use mMousePos instead.
* Remove painter.end() calls (there is no need to call it manually: https://doc.qt.io/qt-5/qpainter.html#end).
* Do not use else after return.
* Return earlier and put return on the next line because eacceptSelection() returns void.
* Store QScreen directly to avoid finding it every iteration.
* Use QMap for mScreenToDpr.
* Use constant iterators.
* Fix headers style.