Stub getBackpack() in MediaInfo, move some checks

This change involves both the scratch-flash and scratch-flash-online
respositories.
This allows more code to be shared between the online, offline, and
open-source editors. This also fixes the 'duplicate' menu option being
absent on costumes in the offline and open-source editors.
Move 'loaders' into MediaInfoOnline
Merge pull request #457 from LLK/base-getbackpack

Stub getBackpack() in MediaInfo, move some checks