Event class enabling/disabling not exported to the user

[deepfire]

While the underlying code allowing for configuration of events being fired is both defined:

• https://github.com/deepfire/reflex-glfw/blob/master/src/Reflex/GLFW.hs#L269
  ..and exercised:
• https://github.com/deepfire/reflex-glfw/blob/master/src/Reflex/GLFW.hs#L329
  ..it hasn't yet been made available to the library user.