Improve favorite mode.

Previously the favorite only mode was implemented by each plugin on its
own as a filter on the bbox selection. This causes a huge overhead in
network requests (most of the data is filtered) and prevents retrieving
all favorite item data at once (without artificially creating bbox
requests). Now the favorite mode is handled by the base plugin directly,
which prevents all bbox queries in favorite only mode and instead queries
the item data directly from the plugin, which only has to implement direct
access to (download of) item data.