Add indication of why Save state is disabled

[tom-seddon]

The Timeline window tells you why the timeline is disabled. There's code to check for save state impediments and code to generate a human-readable string explaining what those impediments are. File > Save state could (somehow...) use this too, to explain why it's currently unavailable.

#137 has long justified letting the save state system languish a bit, but I don't seem to be getting round to it, so it'd be worth improving the existing UI.