filter out empty steps in execution model

A step is empty when every action has no interactions.
add button to clear current execution and mashup

Clears the mashup and redirects user to plan selection.
centrally define JSON Pointers for configuration
tweak composer widget UI
fix sequential IDs to be strings

Fix regression introduced with e7703839005e55893673dbdb28f4c84e68b0881e
change application route #plan to #execution

Clarify that the route addresses a plan's execution.