filemanager: experiment with sea-orm query builder

[mmalenic]

The sea-orm entity query builder could potentially be used in certain sections of the code to make writing queries more straight-forward. sea-orm is compatible with sqlx, however it does not perform any compile-time checks. It would be good to keep compile-time checking of queries, so it might be possible to write queries during the build-step, which are then converted into .sql queries interpreted by sqlx during the main compilation step.

This is dependent on #314.