Watch filesystem to avoid reindexing

[TheFirstMonster]

Is your feature request related to a problem? Please describe.
Reindexing large directories can take a long time and this happens everytime a change to the directory occurs

Describe the solution you'd like
Watch the filesystem for changes and commit those to the db to avoid reindexing the next time the directory is opened

Describe alternatives you've considered (optional)
I am guessing this could be possible via the extension api but I didn't look into it yet

[bpatrik]

This is something that I also want to add myself for a while now.

The design should be something a new job trigger that fires if a folder has changed and runs indexing only on that particular folder.

A further improvement would be to only reindex changed files, not whole folder, but that would be more complex to do.

I wont have time for this, but feel free to send a PR.