Implement and document an approach for removable devices

Tracker originally indexed all removable media by default. This has some issues and we later suggested an 'on-demand' indexing, where apps check for new removable media and show a popup to the user asking if they want to index this device or not. This was implemented internally using the org.freedesktop.Tracker.Miner.Files.Index.IndexFileForProcess D-Bus method wrapped by the tracker_miner_manager_index_file() method.

Since tracker!165 (merged) the tracker_miner_manager_index_file() method isn't available as libtracker-control is made private. This means the existing approach doesn't work with Tracker 3.0.

The fix may be simply this:

Short term, my idea was to export org.freedesktop.Tracker.Miner(.Files) XML files, and let others use gdbus-codegen or GDBusProxy to talk to it.

The following apps that are known to need updating before they will work with Tracker 3.0: GNOME Photos.