Fix filter mode poster screen loading, fixes #210.

Whenever a page of data was loaded we were checking to see if we should show
that list. We should only do that if the data that was just loaded came from
the currently focused list.
Fix audio shuffle, fixes #209.
Allow slideshows to be shuffled, fixes #167.
Remove some commented code/TODOs.
Only allow the universal transcoder for 0.9.7.15 and higher.

The earlier versions have too many issues, like jittery playback.
Add a separate quality pref for non-local content, fixes #207.

This also requires a (welcome) reworking of the way quality is temporarily
overridden. We no longer write to the main quality pref to do an override
and then try to restore it later. Instead we have a separate registry key
for the override, and delete that override when we're done with it (including
at startup, to make sure it can't linger from a previous run).