stop using libglade / port to gtk3

As noted in my mail to openbox mailing list, these to patches first drop libglade, then make the necessary changes to port to gtk3.

GtkBuilder migration is straightforward.
GtkOptionMenu -> GtkComboBoxText move is to make porting to gtk3 easier (this needs gtk+ 2.24), most of the pixbuf changes in src/preview.c for the same reason (see individual commits). Other changes in the first commit (AFAIK) don't go beyond 2.18.

The changes in packing in glade file are also for the sake of gtk3 - they *shouldn't* affect gtk2 (hbox51 removal was mostly to make things cleaner - there didn't seem to be a reason for extra container there). 

The added catch is that http://savannah.gnu.org/bugs/?34506 was only fixed in gettext 0.18.3 (just a few weeks old) and there's a little problem with gtk2 version of glade (http://bugzilla.gnome.org/show_bug.cgi?id=689667). The ui file itself nevertheless works.