Move wxKeyEvent assignment operator out of header

It causes a warning with MSVC code analysis:
"warning C26437: Do not slice (es.63)"
It's unclear if this function even needs to exist, but at least
move it out of line so the warning does not occur for user code.

src/common/event.cpp

@@ -790,6 +790,21 @@ wxKeyEvent::wxKeyEvent(wxEventType eventType, const wxKeyEvent& evt)
     InitPropagation();
 }
 
+wxKeyEvent& wxKeyEvent::operator=(const wxKeyEvent& evt)
+{
+    if ( &evt != this )
+    {
+        wxEvent::operator=(evt);
+
+        // Borland C++ 5.82 doesn't compile an explicit call to an
+        // implicitly defined operator=() so need to do it this way:
+        *static_cast<wxKeyboardState *>(this) = evt;
+
+        DoAssignMembers(evt);
+    }
+    return *this;
+}
+
 void wxKeyEvent::InitPositionIfNecessary() const
 {
     if ( m_hasPosition )