Session Management

[davidrg]

Would be nice if the app saved it state and came back cleanly after, eg, windows updates. Qt has some docs on Session Management which should cover much of this.

Probably this is going to need some sort of session manager thing that windows register themselves with.

Saving and restoring charts is probably going to depend on at least #42 - without this we're limited to restoring the chart windows initial state (whatever options the user chose in the chart options dialog). This alone probably puts it as a post-1.0 feature.