mtp: drop support for libmtp < 0.3.0, add GUdev based detection

The HAL device detection code is still there, and will be until GUdev is
widely available.

This uses the new device management functions in libmtp 0.3.0, which
means multiple devices should work (bug #563544).  Since the device
detection is triggered by the removable media manager now, the 'scan
removable media' menu item now finds available MTP devices (bug #561953).