extensions-app: Only change state if necessary

extensions-app: Only change state if necessary

The `state-set` signal is emitted to change the underlying state,
which can have two reasons:

 1. the user toggled the switch
 2. the extension's `enabled` state changed externally

In the second case, calling enable/disable is pointless at best,
and can mess up the expected state by permanently disabling an
extension that was disabled because of the global kill switch.

Address this by only calling enable/disable if the new state does
not already match the current value of the `enabled` property.

Close https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7305

Fixes: fec523f83f ("extensions-app: Use new 'enabled' property")
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3090>