Re #7482. Add setters to toggle state of live button.

Can now be enabled/disabled & checked/unchecked from the outside.
Re #7462. Remove irrelevant const's in declarations.

Having const on a value parameter in the declaration is meaningless.
It can still be used in the definition where it does have an effect.
Re #7462. Add protection against trying to use with invalid stream.

The Step Scan Interface can only be used with event streams. It now
gives an error message if trying to use it with a histogram listener,
or if StartLiveData fails for any other reason. Previously, there would
be an uncaught exception thrown.
Merge branch 'feature/7462_mwrunfiles_gains_live_button' into develop