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

src/common/event.cpp

@@ -733,9 +733,9 @@ wxPoint wxMouseEvent::GetLogicalPosition(const wxDC& dc) const
 wxKeyEvent::wxKeyEvent(wxEventType type)
 {
     m_eventType = type;
-    m_keyCode = 0;
+    m_keyCode = WXK_NONE;
 #if wxUSE_UNICODE
-    m_uniChar = 0;
+    m_uniChar = WXK_NONE;
 #endif
 }