Add gio::File, gio::FileEnumerator, gio::FileMonitor, gio::Vfs subclasses#1676

fbrouille wants to merge 2 commits intogtk-rs:maingtk-rs/gtk-rs-core:mainfrom

fbrouille:gio_file_subclassfbrouille/gtk-rs-core:gio_file_subclassCopy head branch name to clipboard