codegen: Do not use g_object_class_override_property
codegen: Do not use g_object_class_override_property

Instead, always install our own new property with what we believe the
correct type is.  This avoids a problem with libgee providing properties
on some classes that implement two interfaces having the same property
name with different types.

Fixes bug 666728.

severinh/vala,spidermonkdust/vala,jessevdk/vala,playya/vala,spidermonkdust/vala,frida/vala,xdave/pvala,spidermonkdust/vala,Distrotech/vala,rvu95/vala,rvu95/vala,severinh/vala,GNOME/vala,tjol/vala-tjol,flobrosch/vala-fixes,frida/vala,frida/vala,severinh/vala,frida/vala,rvu95/vala,playya/vala,flobrosch/vala-fixes,jessevdk/vala,flobrosch/vala-fixes,frida/vala,tjol/vala-tjol,Distrotech/vala,playya/vala,GNOME/vala,GNOME/vala,tjol/vala-tjol,Distrotech/vala,jessevdk/vala,GNOME/vala