Merged the garbage collection into master. The garbage collector is a...

Merged the garbage collection into master. The garbage collector is a maintenance stage that runs before the rebuild of thumbnails and must be triggered explicitely.
It removes stale image entries in core db and if enabled also stale thumbnails and face identities from thumbnails and recognition DB.
If configured so, the core DB part of the garbage collector removes stale image entries in core db during the start of digiKam.
Note that cleaning the databases does not necessarily make them smaller as no auto-vacuum is done on the databases. The vacuuming proces differs highly between the three
supported database variants (SQLite, internal MySQL and external MySQL). Thus, currently there is no automatism.

BUGS: 374591 323718
FIXED-IN: 5.5.0