Avoid using deleted widget in idle callback

Use a slightly higher idle priority so callback runs before TLW is deleted,
and ref the widget just to make sure it doesn't disappear. Problem was
introduced in 0388ce8e25

(backport of 9c3900e40d, which modified
9dc7a89ccd, which wasn't backported)

src/gtk/toplevel.cpp

@@ -1193,6 +1193,7 @@ extern "C" {
 static gboolean reset_size_request(void* data)
 {
     gtk_widget_set_size_request(GTK_WIDGET(data), -1, -1);
+    g_object_unref(data);
     return false;
 }
 }
@@ -1221,7 +1222,8 @@ void wxTopLevelWindowGTK::DoSetClientSize(int width, int height)
         {
             gtk_widget_set_size_request(m_wxwindow, m_clientWidth, m_clientHeight);
             // Cancel size request at next idle to allow resizing
-            g_idle_add_full(G_PRIORITY_LOW, reset_size_request, m_wxwindow, NULL);
+            g_idle_add_full(G_PRIORITY_LOW - 1, reset_size_request, m_wxwindow, NULL);
+            g_object_ref(m_wxwindow);
         }
     }
 }