Add new class location of SWT's gtk_widget_get_window

[PeterC-DLS]

In SWT version 4.20, some gtk methods moved to a new gtk3 subpackage so add check and find it in there. Note, this new package was not exported until SWT 4.23 (aka 3.119.0 or v4950) so intervening versions will not work when using OSGi class loading.