Add the dialog to select 'fuentes implicadas' of alternatives
Use human readable names for columns fo select element jtables
Populate 'pobl_actual' and 'caudal_fuentes' when 'poblaciones_implicadas' or 'fuentes_implicadas' are modified
Add a visual identification to show when an alternative is valid
OpenAlternativesExtensions handles when the open alternative is valid

A static member of the extension allows other extension ask it when
the alternative is valid. It is used to manage the control flow of the
application
Epanet tools are disabled if a valid alternatives had not been opened
Fix bug about when SourcesExtension should be enabled