Fix crash on disable when another extension's panel actors are already disposed

[wormuz]

Describe the bug

Dash-to-Panel crashes during disable() when another extension that owns
actors in the shared top panel has already been disabled (and its actors
finalized) before dash-to-panel's turn.

A reliable real-world trigger: the snap firmware-updater notifier runs on a
schedule and calls gnome-extensions disable ubuntu-appindicators. Its
AppIndicatorsIconActor objects are finalized, then dash-to-panel is disabled
and PanelStyle._removeStyles() recursively walks every panel child to restore
styles — touching those dead actors:

TypeError: this._indicator is null
  _waitForFullyReady@.../ubuntu-appindicators/appIndicator.js
  _refreshPanelButton@panelStyle.js
  _restoreOriginalStyle@panelStyle.js
  _recursiveApply@panelStyle.js
  _applyStylesRecursively@panelStyle.js
  _removeStyles@panelStyle.js
  disable@panelStyle.js

Accompanied by repeated:

Object .Gjs_ubuntu-appindicators..._AppIndicatorsIconActor, has been already
disposed — impossible to access it.

Accessing a finalized GObject throws, which spams the journal and aborts the
rest of the style-restore pass (panel left half-restored / flickering).

Root cause

_recursiveApply() / _refreshPanelButton() assume every actor reachable from
the panel is still alive. We don't control the order in which extensions are
disabled, so actors owned by other extensions may already be finalized.

Fix

• Add Utils.isValidActor() — probes whether an actor's underlying GObject is
  still alive (touching a finalized one throws).
• Skip dead actors at the top of _recursiveApply().
• Add a null-parent guard in _refreshPanelButton() for actors detached by
  their owning extension.

No behaviour change for live actors; only already-disposed ones are skipped.

Related to #2439 (same class of disposed-actor crashes).

Environment

• Ubuntu 24.04, GNOME Shell 46, Wayland
• Dash-to-Panel v73