open some classes [API-ish Suggestion/Request]

[SamsTheNerd]

kotlin seems to mark classes as final by default, it'd be nice for BlockEntityMediafiedStorage and its methods to be open so other addons can extend it to add their own mote holding blocks.

Although, it may be better to just make some MoteInventory interface if you wanted to formalize adding new mote holder blocks, but that might be more work than its worth.