SeekBar individual listeners

[dream-1ab]

        SeekBar.onSeek {
            //optional use
            onProgressChanged { seekBar, progress, fromUser ->

            }
            //optional use
            onStartTrackingTouch {

            }
            //optional use
            onStopTrackingTouch {

            }
        }

[dovahkiin98]

the

button.onClick { }

isn't a big difference to be honest
I've already suggested a PR for the EditText, tho now it's in TextView form.
The Seekbar one is new, but maybe there can be more extensions.

[JakeWharton]

There's already a PR for seekbar as well. Perhaps we should codify a pattern for multi-callback listeners.