Bidirectional 'values' binding for slider

Made the ```values``` binding bidirectional for the slider widget.

Also added a new option called ```realtime``` (not sure about the name though), which, when set to true, will make so that the observable bound to ```value``` or ```values``` is updated upon the ```slide``` event instead of ```change```.

I can split this into 2 distinct pull requests, if you are interested in only one of those 2 changes.