Fix the crash that happens when touch device is re-enabled.

This patch allows to destroy the wl_touch interface by calling
wl_touch_destroy when touch device is un-plugged or disabled,
which prevents Wayland from passing a dangling pointer of
WaylandTouchscreen to touch event handlers.

Bug: TC-1729
Merge pull request #308 from joone/fix_touch_crash

Fix the crash that happens when touch device is re-enabled.