Fix retina rendering of titlebar dropdown.

Remove drop shadow filter from the titlebar menu and replace with a second caret to act as a border for separation from the titlebar itself.

BEFORE:
![prepros_app_before](https://f.cloud.github.com/assets/750698/1387157/14d9df68-3b8b-11e3-8477-b77732c45090.png)

AFTER:
![prepros_app_after](https://f.cloud.github.com/assets/750698/1387159/220497f0-3b8b-11e3-8d47-b7d09cbbf473.png)