BREAKING(Accordion): refactor shorthand API to use AccordionPanel

[layershifter]

Is part of #2880.

BREAKING CHANGES

With React 16 we can return an array of elements in render(), this allows us to simplify the shorthand API there.

Changes in this PR affect only the shorthand API.

1. key handling is better now

The usage of AccordionPanel in the shorthand API allows us to make handling of keys more simple and intuitive.

Before

const panels = [
  { title: 'title 1', content: 'content 1' },
  {
    title: {
      key: 'title-2',
      content: <div>title 2</div>,
    },
    content: {
      key: 'content-2',
      content: <div>content 2</div>,
    },
  },
]

<Accordion panels={panels} />

After

const panels = [
  { key: 'panel-1', title: 'title 1', content: 'content 1' },
  {
    key: 'panel-2',
    title: {
      content: <div>title 2</div>,
    },
    content: {
      content: <div>content 2</div>,
    },
  },
]

<Accordion panels={panels} />

2. (Typescript only) Export of AccordionPanelProps

The AccordionPanelProps interface is now exported from AccordionPanel.d.ts.

[codecov-io]

Codecov Report

Merging #2904 into react-16 will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           react-16   #2904      +/-   ##
===========================================
- Coverage      99.7%   99.7%   -0.01%     
===========================================
  Files           162     163       +1     
  Lines          2712    2709       -3     
===========================================
- Hits           2704    2701       -3     
  Misses            8       8

Impacted Files	Coverage Δ
src/modules/Accordion/Accordion.js	100% <100%> (ø)	⬆️
src/modules/Accordion/AccordionPanel.js	100% <100%> (ø)
src/modules/Accordion/AccordionAccordion.js	100% <100%> (ø)	⬆️

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