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/SupportJSClientPreload.js

@@ -0,0 +1,86 @@
+/**
+ * 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.
+ *
+ * @format
+ */
+
+// ==============
+// Preload script
+// ==============
+const {remote, ipcRenderer} = require('electron');
+
+const flipperState = {
+  mainWindowId: 0,
+  isClientInit: false,
+  plugins: null,
+  appName: 'JS App',
+};
+
+ipcRenderer.on('parent-window-id', (event, message) => {
+  flipperState.mainWindowId = message;
+});
+
+function initClient(plugins, appName) {
+  if (flipperState.isClientInit) {
+    return;
+  }
+  if (plugins) {
+    flipperState.plugins = plugins;
+  }
+  if (appName) {
+    flipperState.appName = appName;
+  }
+  if (flipperState.mainWindowId != 0) {
+    ipcRenderer.sendTo(
+      flipperState.mainWindowId,
+      'from-js-emulator-init-client',
+      {
+        command: 'initClient',
+        windowId: remote.getCurrentWebContents().id,
+        payload: {
+          plugins: flipperState.plugins,
+          appName: flipperState.appName,
+        },
+      },
+    );
+    flipperState.isClientInit = true;
+  }
+}
+
+window.FlipperWebviewBridge = {
+  registerPlugins: function(plugins) {
+    flipperState.plugins = plugins;
+  },
+  start: function(appName) {
+    flipperState.appName = appName;
+    initClient();
+  },
+  sendFlipperObject: function(plugin, method, data) {
+    initClient();
+    if (flipperState.mainWindowId != 0) {
+      ipcRenderer.sendTo(flipperState.mainWindowId, 'from-js-emulator', {
+        command: 'sendFlipperObject',
+        payload: {
+          api: plugin,
+          method: method,
+          params: data,
+        },
+      });
+    }
+  },
+  isFlipperSupported: true,
+  initClient: initClient,
+};
+
+ipcRenderer.on('message-to-plugin', (event, message) => {
+  const flipper = window.flipper;
+  if (!flipper) {
+    return;
+  }
+  const receiver = flipper.FlipperWebviewMessageReceiver.receive;
+  const {api, method, params} = message.params;
+  receiver(api, method, JSON.stringify(params));
+});