Implement enable switch in GTK4 viewer

This adds a `state-set` signal on the plugin row, which merely re-broadcasts the signal from the switch. This enables the view to watch for the signal and pass along the desired state to the manager.

The plugin row sets the display of the enable switch whenever the plugin state refreshes. I'm not entirely sure if all states are displayed in the way we want.

Testing Done:
Start viewer and see that auto-loaded plugins are actually enabled, and that others can be enabled/disabled.

Reviewed at https://reviews.imfreedom.org/r/1193/