Add FilesProcessed D-Bus signal to miner

This new signal provides more detailed status info from miners, so that CLI and apps can show the status and reuslts of processing specific files.

Internally, this is exposed as ::file-processed. We batch this signal before sending it over D-Bus for performance reasons.

The miner-fs ::finished-root signal is removed as it's apparently unused, and it overlaps with this one.

This series was previously part of !168 but I decided to separate the FilesProcessed signal from the other changes related with on-demand indexing.