Update mechanism

[Freaky]

We've just had one release fixing a data corruption issue, and another one fixing a CPU use bug, and I have limited reach to inform people of these updates.

At its simplest, I could simply put a link to the releases page on the initial window and encourage people to manually check it from time to time. Easy to do, but equally easy to ignore and makes work for the user.

A custom check shouldn't be that difficult, but is bit of a can of worms with regard to how it's triggered, how the user can opt-out, and even just the basics of how it works - I guess checking a bit of JSON hosted on Github, triggered from a button the user hits.

Automated updates... can probably wait for the Steam release. Which would also be a convenient if somewhat inefficient place for people to donate...