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.

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: [
       {