The switch for enabling/disabling extensions currently bases its
state on the extension's ACTIVE state.
This is problematic in the enabled-but-inactive case, because
activating the switch tries to change the state from off to on,
which leads to enabling an already enabled extension (an obvious
no-op).
The new `enabled` property allows us to do better, and use delayed
state changes where the switch position represents the `enabled`
state, and the trough color whether the extension is ACTIVE.