Adds a setting to enable launching URLs on double-click

Hi Michael,

Nice work on MacPass, it's very well done.

I got used to double-clicking on the URLs in the entry window on my Windows machine (using KeePass 2.x), so I thought I'd implement that here as a setting.  I tried to follow your patterns where possible, and things seem to be working well (I took a swipe at the Deutsch translation too...).

Thanks!
Kurt

![screen shot 2013-07-29 at 9 30 43 pm](https://f.cloud.github.com/assets/3892627/876075/4a027026-f8b8-11e2-898f-3738b3de4b77.png)