compositor/window-drag: Activate drag immediately

We currently try to set the name/accessible-name on the new grab actor first, but the actor is only registered when the grab is activated:

clutter_actor_set_name: assertion 'CLUTTER_IS_ACTOR (self)' failed clutter_actor_set_accessible_name: assertion 'CLUTTER_IS_ACTOR (self)' failed

Fix this by activating the grab right after it is created.

Fixes: b3c80016 ("wayland/xdg-toplevel-drag: Use the new grab API in meta_window_drag_begin")