libtracker-miner: Detach early the processed file in TrackerFileNotifier

This is so priv->pending_index_roots can be prepended/appended without confusing
ongoing crawling on the current folder.
libtracker-miner: Add/handle a "priority" TrackerDirectoryFlag

If that flag is set on a file in the TrackerIndexingTree, the
TrackerFileNotifier will append these when processing, so these
directories will be crawled and inspected as soon as possible
libtracker-miner: Obey the "priority" flag when adding files to processing queues

If a file belongs to a root directory with that flag set, it will get G_PRIORITY_HIGH
at the time of processing the file in TrackerMinerFS queues. Also, make
tracker_miner_fs_check_directory() use G_PRIORITY_HIGH, like check_file() does.
miner-fs: Set the priority flag on mounted volumes.

This is so their content is inspected as soon as possible, even if tracker
is busy crawling over local directories.
Merge branch 'prioritized-crawling'