Merge branch 'sev/xolonium4' into 'master'

Update Xolonium fonts to version 4.0

The new version features revised proportions to improve legibility.
Most notably, the middle bars are lowered, the glyphs in general
are slightly narrower, and the digits in particular are less stretched.

![x3-x4](/uploads/68d6894b2dac14971f2f334d91ca9e66/x3-x4.png)

See merge request !18