Fix retina rendering of titlebar dropdown menu.

Removes -webkit-filter dropshadow from the titlebar menu's dropdown.
Replaces the separation between the caret and background with another
border on the caret.
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.
Merge pull request #360 from studiochris/master

Fix retina rendering of titlebar dropdown.