Add wxGetDPIofHDC() helper function and use it in wxMSW code

No real changes, just replace several pairs of calls to GetDeviceCaps()
with a single helper function call.

src/msw/window.cpp

@@ -4878,9 +4878,7 @@ wxSize wxWindowMSW::GetDPI() const
 
     if ( !dpi.x || !dpi.y )
     {
-        WindowHDC hdc(hwnd);
-        dpi.x = ::GetDeviceCaps(hdc, LOGPIXELSX);
-        dpi.y = ::GetDeviceCaps(hdc, LOGPIXELSY);
+        dpi = wxGetDPIofHDC(WindowHDC(hwnd));
     }
 
     return dpi;