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

.vscode/launch.json

@@ -1,52 +0,0 @@
-{
-  "version": "0.2.0",
-  "configurations": [
-    {
-      "name": "Attach to Running Renderer",
-      "type": "chrome",
-      "request": "attach",
-      "port": 9222,
-      "webRoot": "/",
-      "url": "http://localhost:3000/index.dev.html"
-    },
-    {
-      "name": "Attach to Running Main",
-      "type": "node",
-      "request": "attach",
-      "port": 9229,
-      "sourceMaps": true
-    },
-    {
-      "type": "node",
-      "request": "launch",
-      "name": "Launch Current Jest Suite",
-      "program": "${workspaceFolder}/node_modules/.bin/jest",
-      "args": ["--runInBand", "${relativeFile}"]
-   },
-   {
-    "type": "node",
-    "request": "launch",
-    "name": "Launch Current Script",
-    "args": ["${file}"],
-    "env": {
-      "TS_NODE_FILES": "true"
-    },
-    "protocol": "inspector",
-    "internalConsoleOptions": "openOnSessionStart",
-    "runtimeArgs": [
-      "--require",
-      "ts-node/register"
-    ],
-    "cwd": "${workspaceFolder}"
- }
-  ],
-  "compounds": [
-    {
-        "name": "Attach to All",
-        "configurations": [
-            "Attach to Running Main",
-            "Attach to Running Renderer"
-        ]
-    }
-]
-}