Allow to properly scale the GUI with Qt5

Allow to properly scale the GUI with Qt5

Starting with Qt 5.6, setting the environment variable QT_SCALE_FACTOR
makes everything accordingly bigger. So, if QT_SCALE_FACTOR=1.2, all
text and GUI elements are rendered 20% bigger. However, if an application
does not account for this, everything will also look "blocky".
With this commit, all text and images will be scaled remaining sharp.
This works whether a HiDpi screen is used or not, but is mostly useful
with a HiDpi screen, as all GUI elements are more spaced apart and one
can use the mouse for selecting things without requiring a high precision.