Name struct owning field `owner`

In Weston, we have several structs whose lifetime is strictly controlled by the associated wl_resource: the struct cannot out-live the resource, without exception.

I would propose to rename that struct wl_resource * field to owner to underline this invariant. A struct cannot exist with owner == NULL. It implies that owner is a back-pointer to an object with its own life time.

I think one can find these by grepping for wl_resource_create and inspecting the destructor recorded with wl_resource_set_implementation(). For example, struct weston_surface is not one, because it can exist without its wl_resource.

I feel we should start using this convention at least with new code, and apply it to more owners than just wl_resources.