implement measure popup fixed positionning, documentation to enable on-t...

...he-fly measurement

this implement the new measurement behavior listed in the offer document from 14.06.2012 (MFUsergroup_geoportail_evolutions2013_v1.1.pdf)

#22 AMÉLIORATION DES OUTILS DE MESURES, part a) fixed positioning and b) real time calculation

I decided to use either an Ext.Window or a GeoExt.Popup depending of the positioning chosen by the user, because the GeoExt.Popup has specific styling and behavior which would have been in the way when using a fixed positioning