Added activeIndex prop and onActive event to Carousel

[MarcoMedrano]

This fixes #1187
Using same names than in Tabs

What does this PR do?

Adds activeIndex so the dev can set by code which will be the active child in the Carousel.
Adds onActive so the user can be notified when user swaps or chooses another child in Carousel or when this event is fired automatically by Carousel using autoplay={true}.

Where should the reviewer start?

State has been modified on constructor to get the value from props if there is one:
activeIndex: props.activeIndex || 0,

What testing has been done on this PR?

Just manual test with both Carousel and Tabs.
<Carousel autoplay={true} persistentNav={false} activeIndex={this.state.activeIndex} onActive={index => {console.log(index); this.setState({activeIndex:index})}} >

<Tabs activeIndex={this.state.activeIndex} activeIndex={this.state.activeIndex} onActive={index => {console.log(index); this.setState({activeIndex:index})}} >

How should this be manually tested?

I tested it together a tab to have same behaviors using all combinations of props with activeIndex only, with onActive only with both together and without both.

Do the grommet docs need to be updated?

Yes should be added this doc (copied from https://grommet.github.io/docs/tabs):

activeIndex {number}
Active tab index. Defaults to 0.

onActive {function}
Function that will be called with the active tab index when the currently active tab changes.

Is this change backwards compatible or is it a breaking change?

Yes, it is. If both activeIndex and onActive are excluded, it works as always worked.

• If only activeIndex is included then Carousel shows only that index no matter if the user clicks on Carousel button's or swaps. Even if Carousel has autoplay={true} index is changed only by the property (I determined this as the expected behavior after think a bit :D and check Tabs behavior).

• If only onActive is included then Carousel works as before but notifies the index even if Carousel has autoplay={true}.

• If both are included so last two statements are true.

This is working as Tabs does.

[coveralls]

Coverage decreased (-0.03%) to 26.47% when pulling 07a212f on MarcoMedrano:Fix-#1187 into 4a9fe6f on grommet:master.

[alansouzati]

I don't think we need to call announceFunc here as we already call when setState is invoked.

[MarcoMedrano]

:o Right , mm and not. The announceFunc is only called if this is true ! this.props.hasOwnProperty('activeIndex') If that is false the dev will set the state through componentWillReceiveProps, but they cant explicitly call the announceFunc that is why I called the announceFunc once the dev has updated the props by itself (activeIndex={..}), fired by this.props.onActive(index);
However my code can lead to call twice announceFunc, I think the fix will be:

if (this.props.onActive) {
this.props.onActive(index);
if (this.props.hasOwnProperty('activeIndex')) announceFunc(); }

[alansouzati]

I've added announce every time we set activeIndex

[alansouzati]

thanks for your contribution. I will fix the extra call in my local system.