gtype: add type declaration macros for headers

Add G_DECLARE_DERIVABLE_TYPE() and G_DECLARE_FINAL_TYPE() to allow
skipping almost all of the typical GObject boilerplate code.

These macros make some assumptions about GObject best practice that mean
that they may not be usable with older classes that have to preserve
API/ABI compatibility with a time before these practices existed.

https://bugzilla.gnome.org/show_bug.cgi?id=389585