Ra3xdh activefilter

Dear Guilherme!

I made some changes according the last comment at PullRequest#13 and I ould like to merge it into branch ra3xdh-activefilter. Changes are:

1. All *.svg and *.png moved to bitmaps directory.
2. Implemented automatic removing of Cauer filter topology, if Butterworth, Chebyshev or Bessel are selected. Now error message are not showed.
3. Added QGroupBoxes and Interfaced optimized for laptops with 1366x768 resolution.
4. I will add later some documentation in TeX format to qucs-doc directory. I think it is should describe  underlying mathemathics of filter synthesis algorithms.

Here is screenshot of new interface with QGroupBoxes.

![qucsactivefilter2](https://cloud.githubusercontent.com/assets/4920080/3601150/da27a4d6-0cfe-11e4-8337-deaa1ceed221.png)