Implemented filter audio

Hi,
as echo cancellation was needed I tried to integrate the library as provided by @irungentoo into qTox.
I also implemented a new audio filter setting - the default value is **false**.

TODO:
-
- Makefiles has to be customized for Windows and Mac.
- The libfilteraudio-installer has to be customized for Mac and to be written for Windows.
- It has to be tested.
Why there's a hard dependency on ```filter_audio/filter_audio.h``` being a subdirectory, instead of relying on what system provides in ```/usr/include```, especially when considering that you include script installing ```libfilteraudio.so``` system-wide?