XInput2 has a concept of master and slave devices,
where a slave device is the actual physical device,
attached to a master device representing the cursor or keyboard
focus.
See http://who-t.blogspot.co.uk/2009/05/xi2-recipes-part-1.html
Mouse events were being received from both the master and slave
devices, but we are only interested in events from the master device.
Fixes#533
Scroll deltas are calculated in X11 by comparing the current and
previous absolute values for the scroll axis when a scroll motion
event is received. If the user scrolls whilst the cursor is outside
of the window then an incorrect delta is reported when the cursor
re-enters the window.
Fix this by resetting the last-seen axis values whenever the cursor
re-enters the window.
* For the moment we're still using plain core X11 events
for handling keyboard activity, so remove the XInput2 code for that
* Small refactoring of X11 input handling and documentation fixes
