Refactor multi-step input implemenation to handle special cases

[karthiknadig]

1. Ensure that both QuickPick and QuickInput support cancellation or user pressing escape as CancellationError.
2. Ensure that on step back, right context is passed back to multi-step runner.
3. Ensure that there is a consistent way to handle Back when using QuickPick or QuickInput.
4. hide() on a QuickPick without accept should result in CancellationError.

[eleanorjboyd]

@karthiknadig - thoughts here? Still needed?

[karthiknadig]

This can be closed.