Bookmarks

This branch adds support for bookmarks. See also #45 .
Bookmarks can be added, deleted, modified, tagged. They can also be 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. 

Edit: I should also mention that all the bookmarks and tags are stored in a plain .csv file (separated by an empty line) and that I started from the code by Christian Lindner (https://github.com/charlylima/gqrx).