Makefile: Build ".pyo" files on "make install" (bug 1786)
Create extension for audio and video conversion (bug 1783)

There were 2 similar audio extensions which I joined into the audio_converter extension.
There were 2 similar video extensions which I joined into the video_converter extension.

With this change it's now possible to change the video output format in the advanced
settings to mp4 (default) or avi