provide license information at runtime

[obfusk]

Because the licenses for most recipes (including p4a itself) require some form of attribution, I added an "open source licenses" page to my (webview/flask) app that shows the license text for p4a, python, flask, etc.

Unfortunately I had to manually search for and copy the license files (and must not forget to add new ones if the - possibly transitive - dependencies change).

It would be great if p4a provided a way to automatically get this information at run time. I'd be happy to work on a PR for that (when I have the time).