New event for manual value changes

[jeffposnick]

It would be useful if <paper-slider> fired a new type of event (proposed name: manual-change) when value changes due to manual interaction with the slider.

This new event would allow for

<paper-slider value="{{currentTime}}" on-manual-change="{{handleManulChange}}">

which would invoke the handleManualChange function only when the value changes due to someone interacting with the slider, and not when the bound currentTime variable changes.

This is important if, e.g., the <paper-slider> serves as a media playback scrubber, where it serves the dual-purpose of reflecting the current playback time and allowing someone to seek to a new time manually.

[frankiefu]

fixed by #8