Kill the awful “double call” syntax

[MattiSG]

Currently, actions have to be called twice when used in other actions:

lookup: function(country) {
    return  this.setSelector(country)() // <-- this
                .then(this.submit());
}

This is awful. We need to have something cleaner.

Possible solutions, by order of preference:

1. Add a then property to action wrappers, so that it becomes transparent. See branch experimental-no-double-call.
2. Give all widgets a magic chain/start… property that returns an already-resolved promise and starts the chain. The main question is about the name itself.

[MattiSG]

Implementation idea: make an external module that abstracts calling an array of promise-returning functions sequentially OR a promise-returning function, and use that in every instance where the user could provide a sequence of actions (custom actions in components, feature scenarios).

Allows both readability (use an array, get rid of return and then and their tricks) and extendability (use your own promise-returning function, up to you to ensure you don't forget proper return handling).