Core:
General: removing of all the warnings, use of standard functions,
         better idioms.
Core: predicate for open/close formulas, the intention is not to evaluate
      open formulas.
Parser: corrected typos
Signatures.Figures: removed function symbol "siguiente"
VisualModel: elements can interpret more than one constant.
Gui.EntryFormula: lots of simplifications, key-bindings for inserting
  and deleting elements, update status-bar after some events.
Gui.Board: lots of simplifications, handling of multiple constant
  per element.