GUI: HiDPI support

[shashwatsridhar]

The GUI is not optimised to work on HiDPI screens (for example Retina displays). I noticed this on @marius10p's shared screen. A couple of changes might make it look better on such displays:

• make the GUI HiDPI aware through PyQt (see 1, 2, 3)
• remove all point size font specifications to make GUI HiDPI compatible

Note: this is probably not an urgent issue, but the fix is likely not complicated, so it is a nice-to-have-fixed sort of bug. It doesn't affect the functioning of the software, though.