App icon for Linux (#2116)

Summary:
Fixes https://github.com/facebook/flipper/issues/1369
It seems that electron-builder has an issue when trying to set the app icon for Linux zip builds. A workaround for this is to set `icon` option for `BrowserWindow`.

I created a new PNG file for this, it seems that 64x64 looks the best.

## Changelog

App icon for Linux fixed.

Pull Request resolved: https://github.com/facebook/flipper/pull/2116

Test Plan: ![Screenshot from 2021-03-29 13-50-48](https://user-images.githubusercontent.com/490260/112826545-d00d6e00-9095-11eb-984f-227e923f275e.png)

Reviewed By: mweststrate

Differential Revision: D27461758

Pulled By: passy

fbshipit-source-id: 0be3d6b356d1332e89c3df6c137269900417f9ba

desktop/package.json

@@ -11,9 +11,6 @@
     "asarUnpack": [
       "PortForwardingMacApp.app/**/*"
     ],
-    "deb": {
-      "icon": "icon.icns"
-    },
     "dmg": {
       "background": "dmgBackground.png",
       "contents": [
@@ -46,8 +43,7 @@
       }
     ],
     "linux": {
-      "category": "Development",
-      "icon": "icon.icns"
+      "category": "Development"
     },
     "mac": {
       "category": "public.app-category.developer-tools",

desktop/static/main.ts

@@ -18,6 +18,7 @@ import {
   globalShortcut,
   session,
 } from 'electron';
+import os from 'os';
 import path from 'path';
 import url from 'url';
 import fs from 'fs';
@@ -279,6 +280,12 @@ function createWindow() {
     minWidth: 800,
     minHeight: 600,
     center: true,
+    // The app icon is defined in package.json by default.
+    // When building Linux zip, it must be defined here or else it won't work.
+    icon:
+      os.platform() === 'linux'
+        ? path.join(__dirname, 'icons/app_64x64.png')
+        : undefined,
     webPreferences: {
       enableRemoteModule: true,
       backgroundThrottling: false,