Workout builder supporting messaging during steps

[david]

This got done:

Text events are now supported in the workout builder

The workout builder now supports text events. Currently these only work in zwo (Zwift) files and workouts automatically uploaded using the Zwift integration. Other platforms and file formats get all the messages concatenated together. Here is an example: - First 60^30 Second 120^30 Third 10m 65% That creates a zwo like this: <SteadyState show_avg="1" Power="0.65" Duration="600"> <textevent message="First"/> <textevent message="Second" duration="30" timeoffset="60"/> <textevent mes…