- make sure we set flag in changes when config var changed
- move current deck get/set into backend
- set_config() now returns a bool indicating whether a change was
made, so other operations can be gated off it
- active decks generation is deferred until sched.reset()
- moved 'default to current deck when adding' into prefs
- move some profile options into the collection config, so they're
undoable and will sync. There is (currently) no automatic migration
from the old profile settings, meaning users will need to set the
options again if they've customized them.
- tidy up preferences.py
- drop the deleteMedia option that was not exposed in the UI
