Breaking API change - abandon ItemComponent in favor of renderItem

Summary:
After a fair bit of use, we have concluded that the `ItemComponent` mechanism is not worth the
hassle. Flow has trouble type checking it thoroughly, requiring an 'item' prop is annoying, and it
is very common to need to capture `this` anyway, e.g. for an `onPress` handler. A common pattern was
something like:

  _renderItem = ({item}) => <MyItem foo={item.foo} onPress={() => this._onPress(item)} />};
  ...
  ItemComponent={this._renderItem}

which wouldn't flow check the props and doesn't benefit from reusing components.

If we find some specific patterns that would benefit from the `ItemComponent` pattern, we can create
a new component that provides that API and wraps `FlatList` under the hood.

I'm going to do `SectionList` in a stacked diff.

Reviewed By: bvaughn

Differential Revision: D4625338

fbshipit-source-id: a4901f1c9d77e0115b0b8032b8c210f624e97ea3

Examples/UIExplorer/js/MultiColumnExample.js

@@ -99,7 +99,6 @@ class MultiColumnExample extends React.PureComponent {
         <FlatList
           FooterComponent={FooterComponent}
           HeaderComponent={HeaderComponent}
-          ItemComponent={this._renderItemComponent}
           SeparatorComponent={SeparatorComponent}
           getItemLayout={this.state.fixedHeight ? this._getItemLayout : undefined}
           data={filteredData}
@@ -107,6 +106,7 @@ class MultiColumnExample extends React.PureComponent {
           numColumns={this.state.numColumns || 1}
           onRefresh={() => alert('onRefresh: nothing to refresh :P')}
           refreshing={false}
+          renderItem={this._renderItemComponent}
           shouldItemUpdate={this._shouldItemUpdate}
           disableVirtualization={!this.state.virtualized}
           onViewableItemsChanged={this._onViewableItemsChanged}