Show detected platform on startup and in About popup

[neiljp]

This is an element of #791.

We now have a clear unique platform determination via code in platform_code.py which runs on startup.

For various reasons it would be useful to show this information:

• on the terminal, when the application starts up, possibly with a warning if unsupported
• in the About popup, possibly in a new section at the bottom

Each of these could be separate commits or PRs.