shell-player: remove jump-to-playing-track stuff

We can do this better in the shell.
entry-view: scroll to the playing entry on clicking the column header

This seems like a reasonable thing for the playing column header to do.
It didn't do anything before.
shell: follow the playing track in the track list

Jump to the new playing track when it changes. There is now a
setting that controls this, and an item in the app menu (under
view) for changing it.  While we're here, add an accelerator
key for jumping to the playing track, since that went missing
a while ago.