gtkutils: remove usage of deprecated xthickness and ythickness
GTK+ doesn't support xthickness and ythickness anymore and ignores
the values in its calculations. GTK+ since 3.20 even crash when
gtk_style_context_get() queries the thicknesses.
I removed the properties without providing any replacement and have not
seen any issues with placement of popup menus after that on Ubuntu 14.04
(GTK+ 3.10.8) and newer.