Dialog management cleanup

Summary:
This diff moves the dialogs

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To use the imperative dialog APIs, rather then organising them through reducers which adds a lot of indirection which isn't really needed but hard to follow.

Reviewed By: passy

Differential Revision: D30192002

fbshipit-source-id: ba38b2e700da3e442653786448fcbf85074981ad

desktop/flipper-plugin/src/__tests__/api.node.tsx

@@ -85,6 +85,7 @@ test('Correct top level API exposed', () => {
       "DeviceLogEntry",
       "DeviceLogListener",
       "DevicePluginClient",
+      "DialogResult",
       "Draft",
       "ElementAttribute",
       "ElementData",