GNOME is switching to Tracker 3.x by default (see
Initiatives#17) however Photos
is not ready to switch this cycle.

We want to avoid having two Tracker daemons indexing the same content
as much as possible. This commit means that Photos will start private
instances of Tracker 2.x daemons, and the daemons will be tied to the
lifetime of the Photos app. Distros can disable Tracker 2.x by default
and it will only activate when needed by Photos.

This also allows the Photos Flatpak to run Tracker 2.x daemons inside
the sandbox, ensuring it works on systems which don't have Tracker 2.x
available.

Based on a patch by Yi-Soo An <yisooan@gmail.com>.