Make font selection more amenable to cross platform differences

[aaime]

While setting up the Korean fonts we noticed that two different Linux systems ends up calling the same font in a different way:

versus:

Note the lack of "regular" at the end of the name. The former case happened on Windows, and on a Linux machine where the fonts were just dropped in the Java fonts directory. The latter on a Linux Mint where the fonts where installed via the system utilities, so that they are available to multiple JDKs (dev machine).

Right now fonts are enumerated one by one, it's verbose, somewhat hard to maintain, and sensitive to these differences. On GeoServer 2.17.x the mbstyle module offers the "fontAlternatives" function, which allows to pick every font starting with a certain prefix.

I suggest we:

• Move the function to core, as it's of general usage
• See if we can make it work with multiple prefixes, or allow calling it multiple times
• Check if CSS needs to be modified to support using that function
• Change the styles to just use something like:

font-family: fontAlternatives('Noto Sans'), fontAlternatives('DejaVu Sans'), , fontAlternatives('HanaMin'), , fontAlternatives('Unifont');

This will require a newer version of GeoServer, so release of this data dir should be cut, that is compatible with GeoServer 2.16.0/1/2.