It is wasteful to parse svg icons all the time so lets cache the
result when this make sense in a LRU map.

Change-Id: I95cc317c9301138a9e384d270223ba147a123e59
Reviewed-on: https://gerrit.libreoffice.org/69055
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>