[WIP] scrollable dropdown menus

[n-riesco]

This PR addresses one of the points in plotly#810 :

• provides a helper class ScrollBox (that could be reused to provide scroll boxes elsewhere)

• implements scrollable dropdown menus:

  • scrollbars are added only if needed
  • user can scroll by dragging a scrollbar
  • user can scroll by dragging the scrollbox (menu buttons skip the click triggered by dragend)

TODO

• Add jasmine tests

[n-riesco]

Here's an example to test the functionality:

// Adapted from https://github.com/plotly/plotly.js/pull/770#issuecomment-234669383

var direction = 'down',
    data = [],
    layout = {
        updatemenus: [{
            direction: direction,
            buttons: [{
                method: 'restyle',
                args: ['line.color', 'red'],
                label: 'red'
            }, {
                method: 'restyle',
                args: ['line.color', 'blue'],
                label: 'blue'
            }, {
                method: 'restyle',
                args: ['line.color', 'green'],
                label: 'green'
            }]
        }, {
            y: 0.7,
            direction: direction,
            buttons: []
        }]
    };

for (var i = 0, n = 20; i < n; i++) {
    data.push({
        y: Array.apply(null, Array(10)).map(() => Math.random()),
        line: {
            shape: 'spline',
            color: 'red'
        },
        visible: i === 0,
        name: 'Data set ' + i,
    });

    var visible = Array.apply(null, Array(n)).map((_, j) => i === j);

    layout.updatemenus[1].buttons.push({
        method: 'restyle',
        args: ['visible', visible],
        label: 'Data set ' + i
    });
}

Plotly.plot('graph', data, layout);

[etpinard]

Nice solution! Go ahead and move that PR to the main repo!