Move Pixel class to mediaProperty

This class is a descriptor of a Pixel: its place is with the other
properties.
Rename Pixel to PixelProperties

VideoProperties: rename getPixel to getPixelProperties.
InputFile: avoid to seek when analyse file if just check header
PixelProperties: rename getComponents to getNbComponents
VideoProperties: check if index of stream exist in the file
VideoProperties: add getter to nbFrames
AudioProperties: add getter to nbSamples
VideoProperties: add getDuration

* Could be different from the duration indicated in the header of the
file.
* FileProperties: add doc to getDuration.