* Change 'accessible' to align with React Native.
* Add support for 'importantForAccessibility'.
* Stop event propagation for keyboard-activated Touchables (nested
Touchables now respond the same as when touch-activated).
* Fix whitespace layout of nested Text elements.
* Use 'div' for Text to improve TalkBack grouping.
* Rewrite accessibility docs.
Close#382Fix#408
'Switch' on Web can support custom sizes and colors. To do so,
Web-specific propTypes are introduced: `trackColor`, `thumbColor`,
`activeTrackColor`, and `activeThumbColor`.