Summary:
### Connecting Flipper with JS apps by using electron's BrowserWindow and IPC
1. UI: there is a menu item in Devices tab which opens JS Emulator Launcher Sheet. Here we can configure URL to open and initial size of the window.
2. BrowserWindow, preloaded js: there is SupportJSClientPreload.js which initialize communication between flipper and app via electron's ipc
3. On flipper's side there is src/utils/js-client/serverUtils.tsx which contains most of JS emulator related code
4. Extracting of FlipperClientConnection: since we don't use RScocket to communicate with JS app I extracted needed methods to FlipperClientConnection (located in Client) and partly implemented them in JSClientFlipperConnection (requestResponse is just send a message now, doesn't return actual result)
Reviewed By: jknoxville
Differential Revision: D18572882
fbshipit-source-id: 56d1ca1a60ed2e51329b917021a09382cbb1ceec
