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

include/wx/defs.h

@@ -2289,9 +2289,11 @@ enum wxDataFormatId
     wxDF_MAX
 };
 
-/*  Virtual keycodes */
+/* Key codes */
 enum wxKeyCode
 {
+    WXK_NONE    =    0,
+
     WXK_BACK    =    8, // backspace
     WXK_TAB     =    9,
     WXK_RETURN  =    13,