Rewrite Listings to allow Fetching them and Saving Searches to Browse

Depends on #62

Closes #50

Goals:

• Allow Listings to be fetched from the source instead of always being inside the extension
• Allow Listing and Search data to be saved for later to get similar results
• Keep current extensions working with new listing changes

Notes:

• App will have to be rewritten to include a new screen for choosing a listing after clicking the source, and navigation will have to be nested to allow properly listing nested entries
• On old extensions you still need to call Listing.Item.getListing, it is now nullable and only exists for old extensions