Enable grabbing input for popups, and drag-and-drop. Since the very
switch to using ClutterGrab underneath Wayland grabs will challenge
assumptions in existing code, these had to change in one go. A notable
one is that meta_display_windows_are_interactable() is not 100% true
anymore for xdg_popups, at least not the same.
Another change happening in lockstep is MetaDnD no longer having
to funnel events to Wayland, since the grab triggered by Wayland DnD
is now a cause of "compositor grabs", and will naturally receive events
as long as it hold. while "modal".
A number of ad-hoc checks for grabbing state has also been dropped
from src/wayland/ internals, since again Wayland grabs are a reason
for Clutter grabs, plus the mechanism itself will already take care
of focus loss and restoration.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3420>
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