hide_if_duplicate() unconditionally re-ran handle_output_enter() with
the bar's own wl_output for every non-squashed task, so any app_id or
title event made the task visible on all bars and "all-outputs": false
was effectively ignored. The un-squash path in handle_closed() showed
the replacement task unconditionally, with the same effect.
Track whether the toplevel is actually on the bar's output from the
protocol's output_enter/output_leave events, split the button
show/hide logic out of the protocol handlers, and gate every synthetic
re-show on all-outputs or the tracked output membership.
Fixes#5178
Austin Horstman
cbad42bc9b