[Wp] Fixes a segmentation fault on lablgtk3: do not create a gtk button too early.

src/plugins/wp/GuiConfig.ml

@@ -165,7 +165,7 @@ type mprover =
   | COQ
   | WHY of VCS.Why3_prover.t
 
-class dp_button =
+class dp_button () =
   let render = function
     | NONE -> "(none)"
     | ERGO -> "Alt-Ergo (native)"

src/plugins/wp/GuiConfig.mli

@@ -33,7 +33,7 @@ class dp_chooser :
     method run : unit -> unit (** Edit enabled provers *)
   end
 
-class dp_button :
+class dp_button : unit ->
   object
     inherit Widget.widget
     method update : unit -> unit

src/plugins/wp/GuiPanel.ml

@@ -220,7 +220,7 @@ let wp_panel
     ~label:"Provers..." ~tooltip:"Detect WP Provers" () in
   prover_cfg#connect configure_provers ;
   form#add_label_widget prover_cfg#coerce ;
-  let prover_menu = new GuiConfig.dp_button in
+  let prover_menu = new GuiConfig.dp_button () in
   form#add_field prover_menu#coerce ;
   Gtk_form.register demon prover_menu#update ;
   (* End Form *)