Impleneted the functionality of getting thumbnail

- integrated ffmpeg and libpng lib into project
- gets the thumbnail of the video using the ffmpeg
- saves the get thumbnail as a png file
- for now it is not possible to test this as the main.cpp is not ready for
  that yet.
- There is a commented code that allowed to generate thumbnails also for
  the .avi files, but when I uncomment this cose it crashes on the device,
  but works fine on the simulator