Add WXK_NONE symbolic constant indicating absence of a key.

wxKeyEvent::GetKeyCode() and GetUnicodeKey() return 0 to indicate that the key
code or Unicode character is not available, give symbolic name to this 0 to
make the code using these methods more clear.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65521 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/event.h

@@ -1323,7 +1323,8 @@ public:
         Returns the Unicode character corresponding to this key event.
 
         If the key pressed doesn't have any character value (e.g. a cursor key)
-        this method will return 0.
+        this method will return @c WXK_NONE. In this case you should use
+        GetKeyCode() to retrieve the value of the key.
 
         This function is only available in Unicode build, i.e. when
         @c wxUSE_UNICODE is 1.