fixed compilation with gtk3

[Jiri Moskovcak]

This patch make the wizard compile against gtk3, the changes I have made 
are:

-                gtk_entry_set_editable(GTK_ENTRY(textbox), TRUE);
+                g_object_set(G_OBJECT(textbox), "editable", TRUE, NULL);

-> gtk3 drops a lot of these "helper" function, so to set a property of 
some widget please use g_object_set() which works for both gtk2/3

-    if (k == GDK_Delete || k == GDK_KP_Delete)
+    if (k == GDK_KEY_Delete || k == GDK_KEY_KP_Delete)

-> GDK_KEY_* seems to be the correct name - it also works in both gtk2/3
-> this patch also adds --enable-gtk3 config switch which defaults to 
off, so to recompile wizard with gtk3:

$ make clean && ./configure --enable-gtk3 && make

Nikola, can you please test if it compiles on rhel6?

Thank you,
Jirka