Add support for deleting a shared preference (#1018)

Summary:
This change makes it possible to remove preferences. I also added a `Delete` context menu option to `DataInspector` because I needed it to implement this feature. passy confirmed that it makes sense to add this because delete is a common action.

Fixes https://github.com/facebook/flipper/issues/451
Pull Request resolved: https://github.com/facebook/flipper/pull/1018

Reviewed By: jknoxville

Differential Revision: D21086308

Pulled By: passy

fbshipit-source-id: 551ff0908d5e6c93f58d6012b42e1ee3531de997

desktop/app/src/ui/components/data-inspector/ManagedDataInspector.tsx

@@ -36,6 +36,10 @@ type ManagedDataInspectorProps = {
    * Callback when a value is edited.
    */
   setValue?: (path: Array<string>, val: any) => void;
+  /**
+   * Callback when a delete action is invoked.
+   */
+  onDelete?: (path: Array<string>) => void;
   /**
    * Whether all objects and arrays should be collapsed by default.
    */
@@ -80,6 +84,7 @@ export default class ManagedDataInspector extends PureComponent<
         setValue={this.props.setValue}
         expanded={this.state.expanded}
         onExpanded={this.onExpanded}
+        onDelete={this.props.onDelete}
         expandRoot={this.props.expandRoot}
         collapsed={this.props.collapsed}
         tooltips={this.props.tooltips}