Fix int field of wxCommandEvents generated by menu items in wxMSW.

Set the int field of wxCommandEvent generated by clicking on the menu items
correctly for not checkable items: it is supposed to be -1 and not 0 (which is
the value for checkable but currently unchecked items). This was already the
case for wxGTK and wxOSX and implied by the comments in the code.

Make wxMSW behave like this too and clearly document this behaviour.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69101 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/event.h

@@ -2608,6 +2608,10 @@ public:
         Returns the integer identifier corresponding to a listbox, choice or
         radiobox selection (only if the event was a selection, not a deselection),
         or a boolean value representing the value of a checkbox.
+
+        For a menu item, this method returns -1 if the item is not checkable or
+        a boolean value (true or false) for checkable items indicating the new
+        state of the item.
     */
     int GetInt() const;