Fix #69

Otherwise we end up deadlocking while acquiring the context. Might as
well fail harder & sooner

This is a relicate of per-playlist media uniqueness, which isn't the
case anymore

This field is an implementation detail and is accessed from code for
unit testing purposes only.
The value in database is the one that matters

refs #66

For consistency with media and album
Fix #58

refs #58

For consistency with media
refs #58

refs #58

fix #65

Wait until the refresh query is processed, otherwise we could lose some
changes

refs #65

"Failed to compile/prepare request [SELECT COUNT(DISTINCT id_media ) FROM Media m  LEFT JOIN File f ON m.id_media = f.media_id  WHERE folder_id = ?]: ambiguous column name: folder_id(1)"

refs #55

This allows us to pass a unix path that will be converted as a wine
path, without cluttering the actual code.