Convert g_type_init() to a ctor

Move the guts of g_type_init() into a ctor and turn g_type_init() itself
into a do-nothing function.

g_type_init_with_debug_flags() now ignores its arguments, but it has
always been possible to achieve the same effect via environment
variables.

https://bugzilla.gnome.org/show_bug.cgi?id=686161