(SVideoWriter): computes video framerate

What does this MR do?

• Removes the default framerate value hardcoded to 30 fps in SVideoWriter
• Computes framerate using the timestamps of the first frames in the timeline

How to test it?

• Build & run ExVideoRecorder sample
• Select and play a webcam (or file or stream)
• Click on the Video Recording button
• Select one of the extension .avi and enter a valid path
• Wait few seconds and stop the recording.
• Open the written file with your preferred video player
• Video was recorded with the same framerate as the video timeline

Some results

• video captured using a 640x480 15fps webcam

before	after

Associated issue

Closes #32 (closed)