view: stay fullscreen when view's output is disconnected

view: stay fullscreen when view's output is disconnected

I intended to fix this quite some time ago but didn't get around to it.

I don't think there's any good reason why we need to un-fullscreen a
view when its output is disconnected. We can handle it the same as a
maximized view, and move it to a new output (remaining fullscreen) or,
if all outputs are disconnected, just leave it as-is.

This is helpful for a media-center use-case, where you have just one
view (e.g. Kodi) fullscreen all the time, but the TV might appear to be
disconnected if you switch it to a different source.

Tested with a couple different scenarios:

1. Single output disconnected and re-connected: view stayed fullscreen.
2. Secondary output disconnected: view stayed fullscreen but moved to
   the primary output, and the layer-shell panel on that output was
   hidden as expected. When the secondary output was re-connected, the
   view was moved back (still fullscreen) and the panel on the primary
   appeared again.

Fixes: #864