Add separate x_pointer and x_keyboard classes inheriting from x_device.
    
    This is needed to allow more fine-grained control over X devices without
    using different types.  Using different types is problematic because
    devices act as subjects in the X Flask implementation, and subjects
    cannot be labeled through a type transition (since the output role is
    hardcoded to object_r).
    
    Signed-off-by: Eamon Walsh <ewalsh@tycho.nsa.gov>