client: improve area measurement * Change way of transfering data for display towards using the store where different quantities, symbols can be used. This way we do not depend on the identify tool to find our feature. It avoids the problem that the final click on a snap to point for an area may not hit our feshly drawn feature. At the same time it will get easier to display the measurement result in a different way, like translating the quantity. Right now it is displayed on top of the identified box.