First version of unique value classification for areas

This PR intends to provide a first version of unique value classification for area. I proceed the same way I followed for lines : the UIs still need some polishing.  I particularly think about the windows that are used to edit single configurations were some options will disappear asap. In this PR, we have :

- Unique value recognition for area in the legend library, with tests as usual
- A big work of factorization of what had been made for lines in the UI.
- A quick work of writing so that the UIs are available for area too.

The PR is quite huge mainly because of the factorization work. I've moved about 600 lines of code from PnlRecodedLine to an abstract class, for instance... I may have done it in two parts (legend first, then the UI) but I find it more meaningful this way.