Heptapod development cycles

This introduces the idea of "development cycles", matching Heptapod major.minor versions, and makes clearer that a HDK workspace is suitable for a whole cycle, something that started with the heptapod-switch-new-stable script.

Also the release series get fixed namings, allowing to move them to stable status as well.

The documentation changes (README) strive to make this clearer with a stronger push towards the layout with several workspace in a common directory.

The shortly arriving bump of the stable branch from 0.18 to 0.19 will be made with this already in place.