Allows easier conversion of legacy libraries.
Note that performance will be suboptimal as it must read/write each
library once for each symbol in the library. Doing otherwise would
require modifying the PlugIn API, which probably isn't worth it for
a one-time migration.
Spooling up a full PLAYER in the background takes too long.
This also fixes bugs around how the tables are saved, although
that was just missing code rather than anything architectural.
Fixes: lp:1785436
* https://bugs.launchpad.net/kicad/+bug/1785436