Add wxWindow::GetDPI()

This is simpler to use than wxDisplay(window).GetPPI() which was used
instead of it so far in all ports and can be implemented more
efficiently for wxMSW.

Remove wxGetWinTLW, GetDPI already tries to get the top window.

include/wx/msw/window.h

@@ -99,6 +99,8 @@ public:
 
     virtual bool Reparent(wxWindowBase *newParent) wxOVERRIDE;
 
+    virtual wxSize GetDPI() const wxOVERRIDE;
+
     virtual void WarpPointer(int x, int y) wxOVERRIDE;
     virtual bool EnableTouchEvents(int eventsMask) wxOVERRIDE;