Glib stopped providing any fallback implementations on systems without
memmove() all the way back in 2013. Since then, the symbol is a simple
macro around memmove(); use that function directly now that glib added
a deprecation warning.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/632
Cut down on boilerplate by using the (no longer that) new helper
macros. We don't care about breaking ABI in private libraries, so
use G_DECLARE_FINAL_TYPE even where the class struct used to be
exposed in the header, except for types we inherit from ourselves
(obviously) or where the class exposes any vfuncs (where changes
could affect inheritance in extensions).
