#42 Save/Export Files

[Hugo Giesteira]

This can be a feature or a bug correction.
In the export operations, such as Export Env.Cond., Export SBML, or when saving a file with data from a view, for instance, Simulation reaction values OptFlux allows the user to replace an existent file without asking if he really wants to replace it with a new.
This question must be added before replacing any file.