conf for LineChart

[kum-deepak]

This is not getting covered as part of #1736. It will be taken care of later as a separate activity as it needs some discussions:

• curve, interpolate and tension are related. interpolate and tension will totally be moved to the compat layer.
• _dotRadius is used on mouseover. _dataPointRadius, _dataPointFillOpacity and _dataPointStrokeOpacity otherwise. Documentation is not clear. However, there is a test case covering this. Think of consistent way to move to conf. Maybe something like:

{alwaysShowDots: true, dots: {radius: 4, strokeOpacity: 0.4, fillOpacity: 0.3}, hoverDots: {radius: 5, strokeOpacity: 0.8, fillOpacity: 0.8}}

• Make a decision on xyTipsOn for LineChart #1724 and use that as a policy for similar cases.