Add an About page

[LeafHacker]

To show version, copyright and licence info. Perhaps also lib licences?

[LeafHacker]

We should really do this before 1.0. Having a credits page which shows authors and version is pretty standard, but it's kinda critical that we have a place to list the licenses of all bundled software libs too.

[LeafHacker]

This is probably best achieved using the markdown system that #25 would have removed