GTK caches the settings per display in a static `GArray`, keeping a
reference to the `GdkDisplay` as the key.

However, when closing the display, the corresponding entry is not
removed from the cache in `GtkSettings`.

So when reopening again a `GdkDisplay`, if the new address matches one
of the previously closed display, the cache will return the existing
`GtkSettings` from the cache, which still holds a reference to the old
`GdkScreen` which was freed along the `GdkDisplay`.

To avoid the issue, make sure to remove the `GdkDisplay` and
corresponding `GdkSettings` when closing the `GdkDisplay`.

Also, care must be taken not to recreate the `GdkSettings` and re-add
the `GdkDisplay` to the cache once the display is closed, and make sure
callers of `gtk_settings_get_for_display()` can deal with a returned
value being `NULL` if the display is closed.

Fixes: commit 360a3c16 - "Use a cheaper way to store settings per
display"