Different view of the scheduler

**Is your feature request related to a problem? Please describe.**
I think a different view of scheduling may work better for many cases.

**Describe the solution you'd like**
Rather than "do this at this time" and "start tasks at next opportunity" and "stop at this time" etc which is a bit complex and fragile (though is generally working for me), it may be better to have a more declarative "between time X and time Y, while conditions A, B, C are true, do Things1 else Things2 in this time". So a simple one for me would be "between 4pm and 7pm on weekdays [unconditionally in this case] halve the charge threshold and go into low power mode". In future some of those conditions could be external, eg poll X for grid intensity.

**Describe alternatives you've considered**
Stay with things the way they are or script or ...