ENH: Haptic button events and default devices

Small device changes. Default devices can now be used by not specifying a name or using an empty string. This way examples can run without needing the exact device name, but you can still provide one if you want. OpenHaptics doesn't provide queries for the device names though (could probably do with win32 and our own list of filters).

HapticDeviceClient also emits button press and release events now. So you can queue them to another place. Nice for when you can't afford to miss a button down press or release event on another thread. The VRButtonEvent is now just ButtonEvent and used by both HapticDeviceClient and OpenVRDeviceClient for buttons.