These used to be a sort of default, where python meant python2 and gtk+
meant gtk2. That doesn't make sense now that both are no longer
supported, so rename the modules and the associated metamodules.

python -> python2
gtk+ -> gtk+-2.0
gtkmm -> gtkmm2
meta-gtk-osx-python -> meta-gtk-osx-python2-gtk2
meta-gtk-osx-python-gtk3 -> meta-gtk-osx-python2-gtk3
meta-gtk-osx-core -> meta-gtk-osx-gtk2