Enumerate devices

[RByers]

From the beginning of the brainstorming for this API there was discussion about eventually supporting device enumeration. It looks like we didn't have a GitHub issue to track that though.

My initial naive proposal was to have a simple enumeration method:

partial interface InputDeviceCapabilities {
  // Return all of the (unique) input devices.
  static sequence<InputDeviceCapabilities> getAllDeviceCapabilities();
}