#180 better HiDPI support

[koro7]

I've been enjoying Xournal for a long time and I don't want to miss it. I really appreciate your work.
Unfortunately, with my new setup (HiDPI ultrabook with touchscreen) I experience some difficulties in using Xournal. The menu icons are way too small to read and nearly impossible to click on. Other GTK3 applications scale automatically with Gnome 3.10. As for the menu itself and its fonts this scalling works already flawlessly except the icons below. Please take a look at the attached screenshot.
Thanks a lot
Christof

[Jakub Buriánek]

I have the same issue using Unity 7.2.2