iOS: Enable views to be nested within <Text>

Summary:
Previously, only Text and Image could be nested within Text. Now, any
view can be nested within Text. One restriction of this feature is
that developers must give inline views a width and a height via
the style prop.

Previously, inline Images were supported by using iOS's built-in support
for rendering images with an NSAttributedString via NSTextAttachment.
However, NSAttributedString doesn't support rendering arbitrary views.

This change adds support for nesting views within Text by creating one
NSTextAttachment per inline view. The NSTextAttachments act as placeholders.
They are set to be the size of the corresponding view. After the text is
laid out, we query the text system to find out where it has positioned each
NSTextAttachment. We then position the views to be at those locations.

This commit also contains a change in `RCTShadowText.m`
`_setParagraphStyleOnAttributedString:heightOfTallestSubview:`. It now only sets
`lineHeight`, `textAlign`, and `writingDirection` when they've actua
Closes https://github.com/facebook/react-native/pull/7304

Reviewed By: javache

Differential Revision: D3365373

Pulled By: nicklockwood

fbshipit-source-id: 66d149eb80c5c6725311e1e46d7323eec086ce64

Examples/UIExplorer/TextExample.ios.js

@@ -422,12 +422,13 @@ exports.examples = [
     );
   },
 }, {
-  title: 'Inline images',
+  title: 'Inline views',
   render: function() {
     return (
       <View>
         <Text>
-          This text contains an inline image <Image source={require('./flux.png')} style={{width: 30, height: 11, resizeMode: 'cover'}}/>. Neat, huh?
+          This text contains an inline blue view <View style={{width: 25, height: 25, backgroundColor: 'steelblue'}} /> and
+          an inline image <Image source={require('./flux.png')} style={{width: 30, height: 11, resizeMode: 'cover'}}/>. Neat, huh?
         </Text>
       </View>
     );