protocol: add wl_data_device.{enter,leave}
Data device focus is currently indicated by wl_keyboard focus. However some systems are lacking keyboards, in which case the wl_seat is missing the keyboard capability and clients can't create a wl_keyboard object.
Introduce enter and leave events so that compositors can indicate when a client will receive selection events.
Signed-off-by: Simon Ser contact@emersion.fr Closes: #37