[Development] High-dpi update II - patches ready for initial review

[Sorvig Morten]

We’ve now started the merge process of wip/highdpi to dev. The branch has been squashed and several new changes have been posted to codereview.

The code changes can be roughly divided into four groups:
	1) Revert the existing implementations in the xcb and windows platform plugins
	2) Add cross-platfrom scaling to QtGui
	3) Make necessary platform plugin adaptions
	4) Add/Update tests (of type manual and auto)

The tracking bug is QTBUG-46615, which lists all the changes. I’d like to highlight two of them:
	Patch series top change (for checking out everything): https://codereview.qt-project.org/#/c/114937/
	Main QtGui change: https://codereview.qt-project.org/#/c/114928/

Please note that vacation season may delay responses to review comments. 

Morten