Remove deprecated GTK3 code, fixes #6103 (#6113)

Replaces the following deprecated Gtk classes throughout the codebase by their non-deprecated counterparts. Style, HBox, VBox, HPaned, VPaned, HScrollbar, VScrollbar, VSeparator, HSeparator, Stock, Table, VScale and HScale.

rtgui/placesbrowser.cc

@@ -31,7 +31,8 @@
 
 PlacesBrowser::PlacesBrowser ()
 {
-
+    set_orientation(Gtk::ORIENTATION_VERTICAL);
+    
     scrollw = Gtk::manage (new Gtk::ScrolledWindow ());
     scrollw->set_policy (Gtk::POLICY_NEVER, Gtk::POLICY_AUTOMATIC);
     pack_start (*scrollw);