DummyVideo: rename attribute - refactoring

* Rename _VideoFrameDesc to _videoFrameDesc.
DummyAudio: add setFrame

* Can create a dummyVideo, and set its Frame (like DummyVideo).
* When read the next frame of the object, read silent if no specific
Frame was set, or read the data of the Frame specified.
Merge branch 'dev_refactoringStructures' of https://github.com/cchampet/avTranscoder into dev_DummyAudioSetFrame
Merge pull request #30 from cchampet/dev_DummyAudioSetFrame

DummyAudio: add setFrame