GtkStack: split homogeneous

Split homogeneous into separate hhomogeneous and vhomogeneous
properties. vhomogeneous is what we want for popover menus.
Use GtkStack:vhomogeneous for popover menus

This allows us to get rid of a size group.
Test h/vhomogenenous in teststack

Add separate checkboxes for hhomogeneous and vhomogeneous.