Initial specification of the checkbox launcher language

[Zygmunt Krynicki]

Hey

We had a small call (Sylvain, Pierre, Maciek and me) to discuss the
launcher language.

You can have a look at the initial specification [1] for what we want
to build here.

For some back story. Launchers are declarative programs that run
checkbox in a given way. Currently we have launchers with a set of
ill-designed features that exposed various bits and pieces of
checkbox's development history that is not maintainable or easily
comprehensible.

The design of the new language is based on simple principles:
- Launchers expose useful features of the Session Assistant
- Launchers have sane defaults for interactive testing
- Launchers fit the common cases. Everything else is a custom executable.

With this in place, we can generate correct python code from any
launcher, to use as a starting point in creating custom solutions. New
features, unless they are crystal-clear, should be first prototyped as
executables so that we don't repeat the same mistakes.

Best regards
ZK

[1]  https://docs.google.com/document/d/1SzdjiDISWpYIOemfebNqz0Qb9OcibKm1N-jcDkLFuaY/edit