Don't crash if postURL resolution fails, fixes #198.
Don't allow stream selection for parts without IDs, fixes #200.

Channel content may list the audio stream(s), but if there's no Part id then
there's no way for us to actually select it.
Fix underrun warning dialog, fixes #201.

Now that the video springboard screen doesn't own the video player, use the
registry to track the original quality.
Don't crash if loading an invalid poster row, fixes #202.

It's not exactly clear why the row would be invalid, but it's easy not to crash.
It seems like maybe it was a case where the filter mode was active but had no
content?
Fix a crash during audio playback on 3.x firmware, fixes #199.

Our screen might not exist in URL event callbacks.
Don't hide the shared sections row before it's loaded.

I'm not sure why I ever liked this idea, but every other row starts out
visible and disappears if it's empty. The fact that this one doesn't makes
it more confusing.