Try at #19773: fix OSX tablemodel code when we change the list

Connecting to the will-change signal matches what the code is trying to do.  I
think it may fix the bug, although I don't know because I couldn't reproduce.
bz19797: fix for removing webkit browsers
bz19780: handle DB errors in has_playables() and select_playable_ids()

I didn't know a way to get this to reproduce, but I made a unittest
that should simulate what happened.
bz19766: Re-implement manually setting cover art for items
Fixed redrawing cover art after changes.

After my last change, I noticed that cover art doesn't get updated
until you restart miro.
Re-implemented displaying feed thumbnails for items.