gck provided some APIs that made working with lists of `GObject`s
easier. GLib has for a long time added API that works well enough for
the same use case, like `g_list_copy_deep()`, `g_list_free_full()` and
more recently also `g_clear_list()`, so use those instead.

This commit also bumps the required GLib version to a more modern 2.64
(which is needed for the `g_clear_list()` API).