Move desktop-related code to "desktop" subfolder (#872)

Summary:
Pull Request resolved: https://github.com/facebook/flipper/pull/872
Move all the JS code related to desktop app to "desktop" subfolder.

The structure of "desktop" folder:
- `src` - JS code of Flipper desktop app executing in Electron Renderer (Chrome) process. This folder also contains all the Flipper plugins in subfolder "src/plugins".
- `static` - JS code of Flipper desktop app bootstrapping executing in Electron Main (Node.js) process
- `pkg` - Flipper packaging lib and CLI tool
- `doctor` - Flipper diagnostics lib and CLI tool
- `scripts` - Build scripts for Flipper desktop app
- `headless` - Headless version of Flipper app
- `headless-tests` - Integration tests running agains Flipper headless version

Reviewed By: passy

Differential Revision: D20249304

fbshipit-source-id: 9a51c63b51b92b758a02fc8ebf7d3d116770efe9

README.md

@@ -38,14 +38,14 @@ Both Flipper's desktop app and native mobile SDKs are open-source and MIT licens
 
 This repository includes all parts of Flipper. This includes:
 
-* Flipper's desktop app built using [Electron](https://electronjs.org) (`/src`)
+* Flipper's desktop app built using [Electron](https://electronjs.org) (`/desktop`)
 * native Flipper SDKs for iOS (`/iOS`)
 * native Flipper SDKs for Android (`/android`)
 * Plugins:
-  * Logs (`/src/device-plugins/logs`)
-  * Layout inspector (`/src/plugins/layout`)
-  * Network inspector (`/src/plugins/network`)
-  * Shared Preferences/NSUserDefaults inspector (`/src/plugins/shared_preferences`)
+  * Logs (`/desktop/src/device-plugins/logs`)
+  * Layout inspector (`/desktop/src/plugins/layout`)
+  * Network inspector (`/desktop/src/plugins/network`)
+  * Shared Preferences/NSUserDefaults inspector (`/desktop/src/plugins/shared_preferences`)
 * website and documentation (`/website` / `/docs`)
 
 # Getting started