view: Tidy up view->output/view->fullscreen redundancy

view: Tidy up view->output/view->fullscreen redundancy

A fullscreen view currently has its output specified twice by:
  - struct output *output
  - struct wlr_output *fullscreen

view->fullscreen may also become a dangling pointer if the output is
disconnected, because view_on_output_destroy() clears view->output but
not view->fullscreen.

To eliminate the redundancy and the dangling pointer, let's change
view->fullscreen to a Boolean and rely on view->output to specify the
output.

Along the way, change a few related usages of struct wlr_output to
struct output as well.

No functional change intended.

v2: Don't allow entering fullscreen on disabled output (makes
    conditions for entering/leaving fullscreen symmetric)
v3: Use output_is_usable() helper