Post a motion event after proximity events

This patch splits the meat of xf86libinput_handle_tablet_axis into a helper
function xf86libinput_post_tablet_motion(), to be called right after we send
the proximity in event.
  
Clients that don't handle proximity (e.g. all XI2 clients) don't see the
coordinates we send along with the proximity events. And, for historical
reasons, they may not look at the coordinates in button events. So a device
that comes into proximity and immediately sends a tip down button event
doesn't send a motion event, causing the client to think the tip down was at
whatever the last known position was (before previous prox-out).
  
The practical effect is that when a user tries to draw a few dots, they end up
being connected to each other.