Handlers should be aware of their enabled/disabled state.

[mcwhittemore]

Currently handlers like DoubleClickZoom are unaware of their enabled/disabled state. It is also not possible as a consumer of mapbox-gl to know this state. This makes it impossible to know if you should renable a handler that you have disabled. I think the best place to check for this is in the handler's enable function. A consumer could fake this themselves, but that will lead to some hard to debug bugs when another application or plugin is also enabled and disabling handlers.

I'm currently handling this is gl-draw by disabling before I enable.