Allow more flexable way of saving timeslot item features (gain, cue etc)

[mstratford]

David H. requests that pro-mode gain adjustments are saved with the show plan.

This brings up the side quest of making a nicer way of saving varying meta-data to show plan items (such possible things as the baps automatic play timers and other such funs in future). Currently just adding more db fields like I did for cue points seems like a poor way to scale.

Will require support from MyRadio regardless.