Improve look of favorites preferences

Set a fixed width for the lists, embed the preferences in a
horizontally-scrollable window, and open the preferences window on the
general tab.

rtgui/preferences.h

@@ -272,6 +272,7 @@ class Preferences final :
 
     Gtk::ScrolledWindow *swGeneral;
     Gtk::ScrolledWindow *swImageProcessing;
+    Gtk::ScrolledWindow *swFavorites;
     Gtk::ScrolledWindow *swDynamicProfile;
     Gtk::ScrolledWindow *swFileBrowser;
     Gtk::ScrolledWindow *swColorMan;