Rename the keep-aspect-ratio property to force-aspect-ratio to make it consistent with xvimagesink and ximagesink.

Original commit message from CVS:
* docs/plugins/gst-plugins-bad-plugins.args:
* sys/directdraw/gstdirectdrawsink.c:
(gst_directdraw_sink_class_init):
Rename the keep-aspect-ratio property to force-aspect-ratio to make
it consistent with xvimagesink and ximagesink.