qt: recover main playlist if application crashes or abruptly closes

Akin to the feature text editors have been employing for a while.

The way it works is basically saving the playlist into a temporary file every 5 minutes. This is only done if there have been changes in the playlist, and there is no I/O if not. Probing the change is not done by comparing the temporary file and the playlist, but rather getting change signal from the model itself.

So, read is done asynchronously if there is a recovery file on Qt interface startup and write is done asynchronously if there have been changes in the playlist model.

Request review @chub.