xdg_shell: clarify touch focus during interactive move / resize

The xdg_toplevel.{move,resize} specs say:

If triggered, the surface also will lose the focus of the device (wl_pointer, wl_touch, etc) used for the move/resize.

In case of touch, it's not clear to me what exactly the expected behaviour is. Discussion on IRC showed that there may be need for clarification in spec.

KWin apparently just doesn't send wl_touch events during the move/resize, and sends wl_touch.up at the end of the operation. This seems sensible, but I'm not sure what other compositors do.