Waybar SEGVs in Glib::DispatchNotifier::pipe_io_handler when the MPRIS
module is enabled. The crash is intermittent because it requires a race
between signal emission and object destruction: a playerctl GLib signal
callback (e.g. onPlayerPlay) calls dp.emit(), which writes a pointer to
the Dispatcher into an internal pipe. If the Mpris object is destroyed
before the GLib main loop reads that pipe entry, pipe_io_handler
dereferences a dangling pointer. This typically occurs when a media
player appears or vanishes on D-Bus (browser closing, player quitting)
or during waybar shutdown/config reload.
The root cause is that ~Mpris() calls g_object_unref() on the manager
and player GObjects without first disconnecting the signal handlers that
hold raw `this` pointers. If playerctl holds additional references to
these GObjects, they survive the unref and can still fire signals
targeting the already-destroyed Mpris instance.
Adopt the same cleanup pattern used by the Wireplumber module: call
g_signal_handlers_disconnect_by_data() to sever all signal connections
referencing `this` before releasing the GObjects with g_clear_object().
This guarantees no callbacks can enqueue stale Dispatcher notifications
after teardown begins.
Additionally:
- Clean up old player in onPlayerNameAppeared before replacing it,
fixing a GObject leak and accumulation of dangling signal connections
- Remove duplicate onPlayerStop signal registration (copy-paste bug)
