Revert "backend: Request next track when swiping with 2 fingers"

This reverts commit 99dcbf8e9a97c261aa9620e038e5958dc1a9e975.

We'll need to implement our own ClutterGestureAction for this to work.
backend: Remove mention of the browser plugin
main: Keep the controls visible when seeking with the slider

When seeking with the handle, mark the popup as busy so it's not
hidden, remove the business when releasing the slider handle, or
when an error occurs.

https://bugzilla.gnome.org/show_bug.cgi?id=732711