universal-cp: Remove deprecated GTK+ widget/property use

Don't use Box, VBox, Hbox or Table: use Grid instead. Don't use
HPaned or VPaned, use Paned instead. Don't use properties in Misc,
but those in Widget.

Stop using stock items.

https://bugzilla.gnome.org/show_bug.cgi?id=733594
universal-cp: Workaround for Label alignment issues

It seems Labels in a size group won't correctly align to start with
GTK+ 3.13. Work around this by adding an extra container around each
label, and adding the containers in the size group instead.

https://bugzilla.gnome.org/show_bug.cgi?id=733594