UI: Fix case where cef func may not be present

Due to a particular... plugin we all know and love, the new
QCefWidget::executeJavascript() function may not actually be present as
it should be. So, we have to verify that it's available, and only use it
in that case.

UI/window-basic-main.hpp

@@ -1267,6 +1267,8 @@ public:
 #endif
 };
 
+extern bool cef_js_avail;
+
 class SceneRenameDelegate : public QStyledItemDelegate {
 	Q_OBJECT