DatasStructures: add EssenceDesc

* AudioDesc and VideoDesc inherit EssenceDesc.
* Rename getAudio/VideoCodec to getCodec.
* Rename setAudio/VideoCodec to setCodec.
* remove unused includes.
StreamTranscoder: refactoring - constructors with const ProfileDesc

* Use "const ProfileDesc" instead of "ProfileDesc" when instanciate a
StreamTranscoder.
* Consequences:
     * OutputAudio: setProfile with a const ProfileDesc.
     * OutputVideo: setProfile with a const ProfileDesc.
     * use find instead of [] to access the elements of ProfileDesc.
Transcoder: change way to add Dummy

* 2 new "add(...)" functions, the only ones which can create dummy
streams (from a profileDesc and an EssenceDesc). When the user tries to
create a dummy stream with an empty file name and no EssenceDesc
indicated, there is no add.
* Update comments to fit to this new behavior.
Transcoder: update doc
Transcoder: refactoring - addDummyStream with const EssenceDesc

* Use "const EssenceDesc" instead of "EssenceDesc" when add a
DummyStream.
* Consequences:
     * AudioDesc: copy constructor with const EssenceDesc.
     * VideoDesc: copy constructor with const EssenceDesc.
     * DummyVideo: setVideoDesc from a const VideoDesc (already the case
for setAudioDesc in DummyAudio).
Merge branch 'master' of https://github.com/avTranscoder/avTranscoder into dev_Dummy
Transcoder: delete Dummy arrays in destructor
Merge pull request #28 from cchampet/dev_Dummy

Dummy stream: new way to add