data: Move custom close icon into actions/

data: Move custom close icon into actions/

Adwaita uses ui/ for the corresponding icon, but GTK treats icons
from a resource path as part of the hicolor theme that doesn't
have such a subdir. And as GTK determines subdirs by enumerating
the theme directory, any subdirs that only exist in the resource
are ignored, whoops.

Make sure the icon can be found by moving it to a standard subdir.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1640>