First step in a series to generalize Media Device code, to make adding new Media Device support easier.
This work is far from done (barely started).

- Device detection will be handled by Factory via a MediaDeviceInfo object, passed to MediaDeviceMonitor, which forwards it to whoever needs it (e.g. MediaDevicesApplet).
- This also allows embedding of device-specific information for applet display (e.g. battery %, free space %)
- New devices to be supported inherit classes in mediadevicecollection which abstract core functionality of a media device.

The MediaDeviceCollection stuff itself is turned into a lib.

Note: possibly merging MediaDeviceCache and MediaDeviceMonitor in a future step, since detection is moved to Factory.