tracker-2.3: Add `tracker export` and a way to export user data from Tracker 2.x store

Merged Sam Thursfield requested to merge sam/2.x-export-data into tracker-2.3

I'm not sure if this should also be merged to master. Since this data is moving to app-private databases, it might be better if the apps themselves provided a way to export their data.

I've now tested the exports with the work-in-progrss versions of Files and Photos and the exported data displays as expected, so this should be safe to merge already.

Edited by Sam Thursfield

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