[icons] Use buildResources folder (#771)

This PR updates the application icon folder structure and declarations to be aligned with updated electron-builder documentation. Specifically, we streamline the usage of application icons by:

- Removing direct icon references for each platform in package.json, and replacing with a top-level buildResources directory
- Removing the linux-specific icon so electron-builder can use icon.icns for both Linux and Mac (per the docs)

Per the docs, icons in buildResources will automatically be detected and used accordingly as follows:

- icon.icns -- Mac and Linux (installers and applications)
- background.png and [email protected] (Mac installer backgrounds)
- icon.ico -- Windows (installer and application)

package.json

@@ -93,6 +93,7 @@
     "appId": "com.automattic.wordpress",
     "asar": false,
     "directories": {
+      "buildResources": "./resource/icons",
       "output": "./release"
     },
     "files": [
@@ -110,7 +111,6 @@
       }
     ],
     "mac": {
-      "icon": "./resource/image/mac/app-icon.icns",
       "category": "public.app-category.social-networking",
       "entitlements": "./resource/macos/entitlements.mac.plist",
       "entitlementsInherit": "./resource/macos/entitlements.mac.inherit.plist",
@@ -121,9 +121,7 @@
     },
     "dmg": {
       "title": "WordPress.com Installer",
-      "icon": "./resource/image/mac/dmg-icon.icns",
       "iconSize": 150,
-      "background": "./resource/image/mac/dmg-background/background.png",
       "contents": [
         {
           "x": 486,
@@ -146,7 +144,6 @@
           "ia32"
         ]
       },
-      "icon": "./resource/image/win/wordpress.ico",
       "certificateSubjectName": "Automattic, Inc."
     },
     "nsis": {
@@ -157,7 +154,6 @@
         "deb",
         "tar.gz"
       ],
-      "icon": "./resource/image/linux/512x512.png",
       "executableName": "wpcom",
       "description": "WordPress.com Desktop Client",
       "synopsis": "WordPress.com Desktop client",