Clean up popup styling a little bit

- The menu title is now vertically aligned in the popup
- "back" text is replaced with a &lt; symbol
- Use a multiply unicode character for close symbol
- Made the back/close button a little bit bigger
- Gave the menu-list icons a little hover feedback

Before change:
<img src="https://cloud.githubusercontent.com/assets/545898/2872331/660942e0-d35a-11e3-80e3-43d8a448dc9f.png" width=600/>

After change:
<img src="https://cloud.githubusercontent.com/assets/545898/2872334/84d9eff8-d35a-11e3-8c18-8e9e07a32464.png" width=600/>

It is hard to tell, but the widget icon is slightly darker than the other icons due to it having full opacity. It's to give a slightly better feel for UX.