USB HID: Fixed warnings about unused variables.
USB HID: Split out tablet support into TabletProtocolHandler

 * The previous approach of combining tablet and mouse support
   in MouseProtocolHandler was unintuitive and had the potential
   problem that if a device has multiple HID descriptors, one
   simulating a simple mouse, that one may be found first and
   there is no mechanism of choosing the best handler for a device.
   Now we can look for tablets before looking for mice.
 * Implement pressure, eraser and tilt support. Untested. This
   mostly comes from the patch in #7354, but eraser support should
   work more like intended by the Be API.
 * It appears that my Wacom Intuos 2 cannot be accessed via the
   HID framework, except as regular mouse. So I couldn't actually
   test these changes. I believe what worked before, i.e. QEMU
   absolute mouse position support, should work as good as before.
   Please shout if not!
 * If more recent Wacom tablets /could/ work with these changes,
   they will not, because the HID driver still explicitely ignores
   Wacom devices and the dedicated Wacom driver will not pick up
   newer devices, with its hard-coded and un-generic device support.