Add ability to unhide the window from the renderer

via ipc, for instance if a notification is clicked but the window is
closed/hidden.

// FREEBIE

main.js

@@ -141,7 +141,11 @@ function createWindow () {
     // in an array if your app supports multi windows, this is the time
     // when you should delete the corresponding element.
     mainWindow = null
-  })
+  });
+
+  ipc.on('show-window', function() {
+    mainWindow.show();
+  });
 }
 
 // This method will be called when Electron has finished

preload.js

@@ -15,6 +15,10 @@
     console.log('draw attention');
     ipc.send('draw-attention');
   };
+  window.showWindow = function() {
+    console.log('show window');
+    ipc.send('show-window');
+  };
   /**
   * Enables spell-checking and the right-click context menu in text editors.
   * Electron (`webFrame.setSpellCheckProvider`) only underlines misspelled words;