Restructured under new `samples` and `libraries` folder

Moved `gapi-chrome-apps-lib` folder under a new libraries folder.
Moved the other folders under a new `samples` folder.

This has the clear benefit of having the README shown on GitHub without to scroll down for 27 minutes.

There is a PR waiting for @mangini in the publish scripts repository which might not cover everything and will need thoughtful review.

Note: This is a following of https://github.com/GoogleChrome/chrome-app-samples/pull/281