Add a flag to wxUpdateUIEvent to tell if the item supports the check action

Not all items support check being set in an UpdateUIEvent handler,
so it is nice to provide an API to find out if the item supports it.

Fixes #13369

interface/wx/event.h

@@ -2374,6 +2374,13 @@ public:
     */
     bool GetEnabled() const;
 
+    /**
+        Returns @true if the UI element can be checked.
+
+        @since 3.1.5
+    */
+    bool IsCheckable() const;
+
     /**
         Static function returning a value specifying how wxWidgets will send update
         events: to all windows, or only to those which specify that they will process