AthenaConfiguration: Add event algorithms to AthAlgSeq (if it exists) by default
This MR changes the behavior of addEventAlgo in that we first attempt to add the given algorithm to AthAlgSeq if no sequence is specified. If AthAlgSeq is not there, we fall back to using the internal sequence (current default). This should eliminate the inconsistencies between scenarios when people add algorithms directly to MainServicesCfg (in which case the algorithm ends up in AthMasterSeq) vs when they add them to an empty ComponentAccumulator, which is later merged into MainServicesCfg (in which case the algorithm ends up in AthAlgSeq as one would naively expect).
Closes ATEAM-825