Create WXSendContextMenuEvent() helper function

Put common code from all the different ports into it.

This is not very useful right now, but it will allow to change this
function once, instead of applying the same change to all ports, in the
upcoming commit.

include/wx/window.h

@@ -1513,6 +1513,9 @@ public:
     // Returns true if more idle time is requested.
     virtual bool SendIdleEvents(wxIdleEvent& event);
 
+    // Send wxContextMenuEvent and return true if it was processed.
+    bool WXSendContextMenuEvent(const wxPoint& posInScreenCoords);
+
         // get the handle of the window for the underlying window system: this
         // is only used for wxWin itself or for user code which wants to call
         // platform-specific APIs