add support for lineCap setting for line chart

[liuxuan30]

If user does not draw circles for line chart, the connecting dot seems not very smooth because of default lineCap.Butt
Add support for setting lineCap in this PR. I am not sure if we should put lineCapType into BarLineChartViewBase, currently combined chart and line chart will both have it.

Another thing is when I tried to port the code to drawCubic, I don't see it take effect before/after CGPathAddCurveToPoint, and cubic line seems smooth already, so I don't port it at last. Correct me if I am wrong.

[danielgindi]

It seems like you've introduced duplicate properties...
Also properties of the line I think should be in the data and not in the provider - so you don't have to look around to find line properties scattered across the library :-)

[liuxuan30]

For duplicated properties you mean in combined chart and line chart both have lineCapType? Or the other PR about axis line cap 719?
putting into data class seems easier and can solve the dup, I did think about it, but at that time what I think is something like line property may not be proper to be included in data class.

[liuxuan30]

update to latest interfaces. putting lineCapType into ILineChartDataSet