core: Consider ClutterGrabs checking whether windows are interactive
There's 2 users of this, meta_display_sync_wayland_input_focus() which does already perform these checks on its own, and MetaCursorTracker's update_displayed_cursor() to determine whether it should go with the Wayland client's cursor.
This second check should also consider the existing ClutterGrabs, so make meta_display_windows_are_interactable() handle them for both callers.
Fixes the cursor shown over windows while e.g. there are menus opened.
Close: #2553 (closed)