cursor-shape-v1: Make object inert when underlying cap is destroyed

It was undefined before how long the wp_cursor_shape_device_v1 has any
effect. Let's specify that the object becomes inert when the pointer cap
goes away or the tablet tool is removed. In those cases the client has
to create a new pointer/tablet tool, and also a new cursor shape device
when the cursor caps or a new tablet tool reappears.

Signed-off-by: Sebastian Wick <sebastian.wick@redhat.com>
Closes: #186