Summary:
This adds new development feature to React Native that provides information
about selected element (see the demo in Test Plan).
This is how it works:
App's root component is rendered to a container that also has a hidden layer called
`<InspectorOverlay/>`. When activated, it shows full screen view and captures all
touches. On every touch we ask UIManager to find a view for given {x,y} coordinates.
Then, we use React's internals to find corresponding React component. `setRootInstance`
is used to remember the top level component to start search from, lmk if you have a
better idea how to do this. Given a component, we can climb up its owners tree
to provice more context on how/where the component is used.
In future we could use the `hierarchy` array to inspect and print their props/state.
Known bugs and limitations:
* InspectorOverlay sometimes receives touches with incorrect coordinates (wtf)
* Not integrated with React Chrome Devtools (maybe in followup diffs)
* Doesn't work with popovers (maybe put the element inspector into an `<Overlay/>`?)
@public
Test Plan:
https://www.facebook.com/pxlcld/mn5k
Works nicely with scrollviews
