Fix todo in redshift-gtk: toggle_item is checkbox that follows the icon

I have made `toggle_item` into a `CheckMenuItem` with the label "Enabled".
Whenever the status is updated `toggle_item`'s active state is set accordingly.
This lead to an loop where setting the active status of `toggle_item` triggered
the active event that calls `toggle_cb` which changes the status.
I resolved adding a semaphore (which a thread cannot enter multiple times
concurrently) that is acquired when change the active status if `toggle_item`,
but also — non-blockingly — around the instructions in `toggle_cb`.