A single `delegate_dispatch2!` replaces all other delegate macros.
When wayland-rs updates the definition of `Dispatch`, this macro will be
unnecessary, and `Dispatch` type bounds should become inferable by the
type system.
`Dispatch` is now implemented for the user-data type, so `smithay` is
able to provide blanket implementations as long as the user data is a
type owned by `smithay`. Therefore, `GlobalData` replaces `()` as a user
data (`smithay-client-toolkit` already did this), and udata that is
simply a type like `Weak<WlSurface>` is wrapped.
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