New sliding-window seed finder in CbmTaskBuildRawEvents with QA

Implemented a new seed finder based on the old "sliding-window" event builder "CbmBuildEventsSimple". Seed times are obtained when the number of digis within a sliding time window exceeds a threshold. The purpose is to enable seed finding with a reference detector which has large background noise.

QA for seed finder developed. When active, it outputs diagnostic information about the constructed event seeds, such as the seed time, the total number of digis in the seed window, the number of noise digis and the cross-contamination by other events.

Corresponding instructions were added to /reco/eventbuilder/digis/README.md.

Fully operational (tested on MC data with noise) and ready for merge in my opinion. Suggested reviewer: @v.friese

This MR also encompasses all commits from !462 (merged) and !452 (closed) and hence renders them obsolete.