Added top level menu shortcuts, pull labels from messages.json (#1702)

* Added top level menu shortcuts (Fixes #1688 and #1695).

The ampersand (&) character in front of a letter in a menu label
indicates that the letter that follows '&' will be used as a keyboard
shortcut letter to access this menu. In Windows/Linux, the default
shortcut combination is Alt+<letter>.

* Use non-hardcoded menu labels.

The menu labels were hardcoded in English.
We should not be using plain strings right in the source code, but
pulling them from the `messages.json` files instead.

_locales/en/messages.json

@@ -1,4 +1,24 @@
 {
+    "mainMenuFile": {
+      "message": "&File",
+      "description": "The label that is used for the File menu in the program main menu. The '&' indicates that the following letter will be used as the keyboard 'shortcut letter' for accessing the menu with the Alt-<letter> combination."
+    },
+    "mainMenuEdit": {
+      "message": "&Edit",
+      "description": "The label that is used for the Edit menu in the program main menu. The '&' indicates that the following letter will be used as the keyboard 'shortcut letter' for accessing the menu with the Alt-<letter> combination."
+    },
+    "mainMenuView": {
+      "message": "&View",
+      "description": "The label that is used for the View menu in the program main menu. The '&' indicates that the following letter will be used as the keyboard 'shortcut letter' for accessing the menu with the Alt-<letter> combination."
+    },
+    "mainMenuWindow": {
+      "message": "&Window",
+      "description": "The label that is used for the Window menu in the program main menu. The '&' indicates that the following letter will be used as the keyboard 'shortcut letter' for accessing the menu with the Alt-<letter> combination."
+    },
+    "mainMenuHelp": {
+      "message": "&Help",
+      "description": "The label that is used for the Help menu in the program main menu. The '&' indicates that the following letter will be used as the keyboard 'shortcut letter' for accessing the menu with the Alt-<letter> combination."
+    },
     "loading": {
       "message": "Loading...",
       "description": "Message shown on the loading screen before we've loaded any messages"

app/menu.js

@@ -7,7 +7,7 @@ function createTemplate(options, messages) {
   const openForums = options.openForums;
 
   let template = [{
-    label: 'File',
+    label: messages.mainMenuFile.message,
     submenu: [
       {
         role: 'quit',
@@ -15,7 +15,7 @@ function createTemplate(options, messages) {
     ]
   },
   {
-    label: 'Edit',
+    label: messages.mainMenuEdit.message,
     submenu: [
       {
         role: 'undo',
@@ -47,7 +47,7 @@ function createTemplate(options, messages) {
     ]
   },
   {
-    label: 'View',
+    label: messages.mainMenuView.message,
     submenu: [
       {
         role: 'resetzoom',
@@ -80,6 +80,7 @@ function createTemplate(options, messages) {
     ]
   },
   {
+    label: messages.mainMenuWindow.message,
     role: 'window',
     submenu: [
       {
@@ -88,6 +89,7 @@ function createTemplate(options, messages) {
     ]
   },
   {
+    label: messages.mainMenuHelp.message,
     role: 'help',
     submenu: [
       {