Update Xolonium webfonts
This branch updates the webfonts to Xolonium 4.0.
Note that I have deleted the .svg format, because it is deprecated.
In return, I have added the new .woff2 format, which offers better compression.
Also note that I have removed Arial and Helvetica as fallbacks, resulting in:
font-family:Xolonium,sans-serif;
I consider this preferable, because it falls back to the default sans-serif font,
which should be the "best" font as determined by the system or the user.
I can change it back if desired.