[FEATURE] Clear caches with the clearCache=system

This commit adds the fluidcontent cache to the cache group 'system'.
This means, that the cache will be cleared when you use the
"Flush system caches" command.
Currently you need to go into the installer to clear this cache,
which gets tedious over time.
Merge pull request #188 from mneuhaus/patch-1

[FEATURE] Clear caches with the clearCache=system