Add Menu File with Open, Save, Save As and Exit

[alixdamman]

[gdementen]

we need to be able to load a second .h5 along side the current session (see #114).

[alixdamman]

Question: Why MappingEditor and ArrayEditor are QDialog and not QMainWindow?
Because of Spyder?

I'm asking because it would be much more easier and suitable to use a QMainWindow instead of QDialog to add a menu bar.