Improve small parts of UI

Add separators to right click menu
Make the main Gtk::Paned split more nicely

This is the UI on default settings now.
![screenshot from 2014-07-25 23 49 27](https://cloud.githubusercontent.com/assets/6528368/3710317/02a841ac-1481-11e4-9cdf-e48dc5362a28.png)