Improve support of high DPI displays

[wmayer]

There is an interesting Qt blog about support of high DPI displays: https://blog.qt.io/blog/2016/01/26/high ... in-qt-5-6/
An interesting fact is that it can be tested on any hardware, i.e. also on low resolution displays. All what must be set is the environment QT_SCALE_FACTOR=1.5 (or any other value) before starting the application.

This way I could reproduce the issue issue #0003130 where the rubberband has an offset.

[sgrogan]

I guess the high resolution difficulties are general mac problems. So not something specific to the conda-packages. Anyway it would be nice to fix them.

I see some commits to master from wmayer.https://github.com/FreeCAD/FreeCAD/commits/master
@looo are you able to provide a build for this? Great work. When you are ready I would like to uplaod an OSX build to the releases page as a release candidate, let me know when you are ready.

[wmayer]

Maybe doing a 0.18.1 release with some backports?

Since we split the 0.18 branch I have pushed a couple of commits to the master branch. Some of them can be backported to v0.18:
git commit a14b99e77
git commit 2f2d50535
git commit bb6e4e6ad
git commit 23ecb8eac
git commit ca7770b80
git commit 51fcdd2c0
git commit 805ccd8deb
git commit 347156403
git commit 094dda5900d
git commit 925cffc1535
git commit 7dfeb801a