Make the autoscroll widget use Html display.
Implementation of the content of the MeasureSpace.
Changes to Measure extensions.

BaseEditor, BaseEngine, BaseMonitor now keeps a reference to the declaration which built them.
All Measure extension now have an id which must be unique a name. Manifest can now declare several measure tools of the same kind.
MeasurePlugin now has a reference to the workspace if it does exist.
MeasureSpace  has a new property 'dock_area' returning the main dock area of the content.