When MouseKeys are activated, keyboard devices may generate fake mouse button
events through XKB. Let's get then running through the appropriate paths, i.e.
as XI events on the correct device.

To make matters more fun, ProcessOtherEvents drops events if the DIX device
state cannot be updated accordingly, i.e. all button events from keyboard
devices.
Hence we need to get the paired MD for the device in XkbDDXFakeDeviceButton,
and post the event through the paired MD (usually the VCP).

Removes now-unused ddxFakeBtn.c.

Note: this patch only half-arsedly fixed button events, motion events are a
more complicated matter.