Add View -> Debug Log to menu bar

Just send an event from the main process to the renderer,
The latter routes it the appropriate view method.

For now it's a no-op unless the main window exists and it is showing the inbox,
which will be addressed in a future commit.

// FREEBIE

main.js

@@ -149,6 +149,12 @@ function createWindow () {
   });
 }
 
+function showDebugLog() {
+  if (mainWindow) {
+    mainWindow.webContents.send('debug-log')
+  }
+}
+
 // This method will be called when Electron has finished
 // initialization and is ready to create browser windows.
 // Some APIs can only be used after this event occurs.
@@ -169,6 +175,9 @@ app.on('ready', function() {
     template[3].submenu[3].click = function() {
       mainWindow.show();
     };
+    template[2].submenu[0].click = showDebugLog;
+  } else {
+    template[1].submenu[0].click = showDebugLog;
   }
 
   const menu = Menu.buildFromTemplate(template);