Add --2to3 exporting capabilities to `tracker3 export`

It sounds useful to be able to migrate portions of the Tracker 2.x data without depending on Tracker 2.x itself. This MR adds a --2to3 option to tracker3 export which opens the 2.x database location and performs direct SQL queries to extract information.

So far only files-starred option is handled to ease Nautilus migration. It works in the same fashion that tracker export --type files-starred command in 2.x. Optionally a --keyfile argument can be used so it exports in that format.