DVB Header information for video streams

Changes:
* Added header information from PMT tables for MPEG-2 and H.264 Streams: (fps, Chroma, resolution, Aspect Ratio). 
* This information is now shown during the scanning process
* Reviewed initial load: The default video and audio format are now taken from the default channel to be tunned (instead of hardcoded MPEG-2)
* Review the switching channel approach to be able to change the header information each time a channel is switched.
* The updated header information includes: AvgTimePerFrame, Aspect Ratio, initial resolution, and interlace flag
* Some additional changes previous to supporting DVB-T2
 
Known issues:
* The resolution and aspect ratio are not informed according to the DVB specifications in most of the channels. The TARGET_BACKGROUND_GRID descriptor should be mandatory for all resolutions different than 720x576 (for pal). I could only find one multiplex informing that in the PMT. It would be important to find a way for informing that resolution during the scanning process so each time we switch a channel, we start with the correct resolution, instead of changing the screen size two or three times.

Note: this PR is linked to a previously closed one. Check PR #50 for previous discussion.