#75 Empty archives can easily raise exceptions

[Ark]

If you have opened an empty archive, virtually everything you can interact with is prone to exceptions. The corresponding controls should be either disabled (the "Properties" entry in the "File" menu, for example) or made fault-tolerant (the controls in the "Preferences" window, for example).

[Ark]

Considering that fixing this might involve a lot of changes nearly everywhere and, besides that, changing to an empty archive doesn't break things by itself, I think we can put it off a bit. Any objections?

[Benoit Pierre]

See my last comment on [bugs:#73].