Windows QPA: add support for MouseDoubleClickDistance

Query the double click distance using the windows GetSystemMetric.
See https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-getsystemmetrics

Change-Id: I6198a38ab1a6216286897f8bdb305f334b7b148e
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/windows/qwindowstheme.cpp

@@ -466,6 +466,8 @@ QVariant QWindowsTheme::themeHint(ThemeHint hint) const
             result = int(scrollLines);
         return QVariant(result);
     }
+    case MouseDoubleClickDistance:
+        return GetSystemMetrics(SM_CXDOUBLECLK);
     default:
         break;
     }