Prepare for FairRoot v18.8

Use the same target names as provided by FairRoot with version v18.8.

Add the new target FairRoot::Online which is a new library conatining the online related code which was previously part of the Base library. In case of older FairRoot versions the target is only an alias for the target FairRoot::Base such that the same dependencies already work with FairRoot v18.6 which is our current default version.

Add the new target FairRoot::Online in all CMakeLists.txt where it is needed.