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

docs/extending/testing.md

@@ -97,7 +97,7 @@ TEST(MyFlipperPluginTests, testDummy) {
 ## Testing the Flipper Desktop Plugin
 
 Tests should be put in the `__tests__` directory of your plugin sources, and be created using Jest.
-An example test suite can be found [here](https://github.com/facebook/flipper/blob/master/src/plugins/layout/__tests__/ProxyArchiveClient.node.tsx).
+An example test suite can be found [here](https://github.com/facebook/flipper/blob/master/desktop/src/plugins/layout/__tests__/ProxyArchiveClient.node.tsx).
 
 Flipper exposes an API to generate unit tests that can verify _regressions_ and real life scenarios.
 To generate a unit test: