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

desktop/static/electron-stubs.notjs

@@ -0,0 +1,37 @@
+/*
+ *  Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ *  This source code is licensed under the MIT license found in the LICENSE
+ *  file in the root directory of this source tree.
+ *
+ */
+{
+  remote: {
+    process: {
+      env: {},
+    },
+    getCurrentWindow: function() {
+      return {
+        isFocused: function() {return true;},
+        on: function() {return true;}
+      };
+    },
+    app: {
+      getVersion: function() {return global.__VERSION__ || '1';},
+      getName: function() {return '';},
+      getAppPath: function() {return process.cwd();}
+    },
+    shell: {
+      openExternal: function() {}
+    },
+    Menu: {
+      buildFromTemplate: function() {
+        return {items: []}
+      },
+      setApplicationMenu: function() {}
+    }
+  },
+  ipcRenderer: {
+    on: function() {return true;}
+  },
+}