Bookmarks

This branch adds support for bookmarks.
Bookmarks can be added, deleted, modified, tagged. They can also filtered by tags.
Each tag has it's own color and it is shown in the FFT screen. Clicking on its label will tune to the corresponding frequency. The modulation mode is stored but it is not automatically restored.

![screenshot from 2014-05-11 02 10 21](https://cloud.githubusercontent.com/assets/5714076/2937412/b76a5894-d8a1-11e3-8384-d6a4580045f8.png)

If you are interested in this feature please let me if there is any improvement I can do.