server: add wl_global_remove
This commit adds a new wl_global_remove function that just sends a global remove event without destroying it. See 1 for details.
Removing a global is racy, because clients have no way to acknowledge they received the remove event. There's no way to fix this, since wl_registry is frozen.
It's possible to mitigate the issue by sending the removal event, waiting a little and then destructing the global for real. The "wait a little" part is compositor policy.
Patch for wlroots using this MR: https://github.com/swaywm/wlroots/pull/1765