Move WizardScrolled widget setup to .ui file

This introduces a critical warning at runtime:

(gnome-boxes:15496): Gtk-CRITICAL **: gtk_range_get_adjustment:
assertion 'GTK_IS_RANGE (range)' failed

But its either gtk+ or vala bug:

https://bugzilla.gnome.org/show_bug.cgi?id=721407
Move WizardSource widget setup to .ui files