[Linux] add cache for font family -> font path mapping

As FcFontMach is quite an expensive call the results of getFontFile() are
now cached in an std::map if either a ttf file is packed with the app or
if a font file has been found by using fontconfig
Merge branch 'master' into feature-linux-fontcache
fontcache map is now static, as we do not need external linkage for it.
Merge pull request #2150 from Weeds/feature-linux-fontcache

[Linux] add cache for font family -> font path mapping