Currently, there is no quick way to find whether and element is already
part of a list store, except for manually writing a for-loop and calling
`g_list_model_get_item()` and breaking when you find the item.
This is mostly just a small API addition to support this use case.
Fixes https://gitlab.gnome.org/GNOME/glib/issues/1011
These are alternatives to g_file_info_{get,set}_modification_time(),
which will soon be deprecated due to using the deprecated GTimeVal
type, which is not year 2038 safe.
The new APIs take a GDateTime instead.
Signed-off-by: Philip Withnall <withnall@endlessm.com>
Helps: #1438
