[ReactNative] refactor the inspector

Summary:
The `InspectorOverlay` component was getting unwieldy, so I broke it into three components:

- Inspector
- InspectorOverlay
- InspectorPanel

and added @flow types.

The inspector was also living under the `ReactIOS` directory, and I moved it
up into the `Libraries` directory, as the inspector will soon be usable [on
Android](https://phabricator.fb.com/D2138319).

All features of the inspector should remain functional, with the addition of
one feature:

- you can toggle "touch to inspect" by tapping the "Inspect" button at the
  bottom of the inspection panel. When inspection is disabled, the panel remains, but you can interact with
  the app normally without touches being intercepted

@public

Test Plan:
Open the inspector:

- touch to inspect things, verify that margin, padding, size and position are
  reported correctly, and that the component hierarchy is navigable.
- tap the "Inspect" button, and verify that you can interact with the app
  normally.

{F22548949}

[Video of toggling inspection](https://www.latest.facebook.com/pxlcld/mrs9)

Libraries/Inspector/resolveBoxStyle.js

@@ -0,0 +1,59 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @providesModule resolveBoxStyle
+ * @flow
+ */
+'use strict';
+
+/**
+ * Resolve a style property into it's component parts, e.g.
+ *
+ * resolveProperties('margin', {margin: 5, marginBottom: 10})
+ * ->
+ * {top: 5, left: 5, right: 5, bottom: 10}
+ *
+ * If none are set, returns false.
+ */
+function resolveBoxStyle(prefix: string, style: Object): ?Object {
+  var res = {};
+  var subs = ['top', 'left', 'bottom', 'right'];
+  var set = false;
+  subs.forEach(sub => {
+    res[sub] = style[prefix] || 0;
+  });
+  if (style[prefix]) {
+    set = true;
+  }
+  if (style[prefix + 'Vertical']) {
+    res.top = res.bottom = style[prefix + 'Vertical'];
+    set = true;
+  }
+  if (style[prefix + 'Horizontal']) {
+    res.left = res.right = style[prefix + 'Horizontal'];
+    set = true;
+  }
+  subs.forEach(sub => {
+    var val = style[prefix + capFirst(sub)];
+    if (val) {
+      res[sub] = val;
+      set = true;
+    }
+  });
+  if (!set) {
+    return;
+  }
+  return res;
+}
+
+function capFirst(text) {
+  return text[0].toUpperCase() + text.slice(1);
+}
+
+module.exports = resolveBoxStyle;
+