Fix GetBestSize() for hidden controls in wxGTK

gtk_widget_get_preferred_size() return zero size if the GTK widget is
hidden, so show it temporarily in order to find its real preferred size.

Add a unit test checking that the best size of a hidden wxChoice is now
determined correctly.

Closes https://github.com/wxWidgets/wxWidgets/pull/1587

src/gtk/control.cpp

@@ -341,9 +341,21 @@ wxSize wxControl::GTKGetPreferredSize(GtkWidget* widget) const
 #ifdef __WXGTK3__
     int w, h;
     gtk_widget_get_size_request(widget, &w, &h);
+
+    // gtk_widget_get_preferred_size() just returns 0 if the control is hidden,
+    // so we have to temporarily show the widget before calling it to get
+    // something useful from it, if it's currently hidden.
+    // So workaround this case.
+    const bool wasHidden = !gtk_widget_get_visible(widget);
+    if ( wasHidden )
+        gtk_widget_show(widget);
+
     gtk_widget_set_size_request(widget, -1, -1);
     gtk_widget_get_preferred_size(widget, NULL, &req);
     gtk_widget_set_size_request(widget, w, h);
+
+    if ( wasHidden )
+        gtk_widget_hide(widget);
 #else
     GTK_WIDGET_GET_CLASS(widget)->size_request(widget, &req);
 #endif