Save list should only request metadata to avoid long request times

[darkf]

Sub-issue of #16.

Currently save list requests also request the full associated save data, which is quite slow. It should only request the save metadata to display a list of saves.

[darkf]

I'm not sure if querying only parts of objects is supported by IndexedDB, so we might have to split the DB into two object stores: savemeta (for metadata) and saves (for savegame objects), linked together relationally with an ID index.