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.
Before allocating a new filterer, you should check if there already is one, and if so, garbage collect it. (callId can be reused, so this is necessary.)