Improve drag and wheel of legend scrollbars
Improve drag and wheel of legend scrollbars

* Now drag events make the scrollbar follow the mouse position.

* Now wheel events move the scrollbar position as fast as drag events.

plotly/plotly.js,asolagmbh/plotly.js,etpinard/plotly.js,aburato/plotly.js,plotly/plotly.js,iongroup/plotly.js,plotly/plotly.js,plotly/plotly.js,iongroup/plotly.js,etpinard/plotly.js,asolagmbh/plotly.js,iongroup/plotly.js,asolagmbh/plotly.js,aburato/plotly.js,etpinard/plotly.js