BETA5
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http://trac.openlayers.org/wiki/Control/OverviewMap
http://www.developpez.net/forums/d816277/applications/sig-systeme-dinformation-geographique/ign-api-geoportail/fonctionnement-overviewmap/#post4690112
http://trac.openlayers.org/browser/trunk/openlayers/lib/OpenLayers/Control/OverviewMap.js
http://workshops.opengeo.org/openlayers-intro/controls.html#controls