In case the DB provides no data, try to fetch the properties directly
from the file using the new OnDemandIndexer class.

If required, start the extractor in the background, fetch UserMetaData
and wait for the extractor, then unite the results.

BUG: 410114
Depends on D25248

1. suspend baloo (balooctl suspend)
2. copy a file with properties (e.g. PNG) to a included location cp ~/foo.png ~/foo_2.png
3. balooshow ~/foo.png ~/foo_2.png should only display "Cached properties" for the original file
4. dolphin displays properties for both files (in the side panel)