Whether to allow instances of proxy target to be created.
This effectively means whether the given enhancer can be used.
By default all enhancers that work on-demand ([iface@Clapper.Extractable], [iface@Clapper.Playlistable]) are allowed while enhancers implementing [iface@Clapper.Reactable] are not.
Value of this property from a `GLOBAL` [class@Clapper.EnhancerProxyList] will carry over to all newly created [class@Clapper.Player] objects, while altering this on `LOCAL` proxy list will only influence given player instance that list belongs to.
Changing this property will not remove already created enhancer instances, thus it is usually best practice to allow/disallow creation of given enhancer plugin right after [class@Clapper.Player] is created (before it or its queue are used).