[Tracker] support for nfo:frameRate

[Alexey Fisher <bug-track fisher-privat net>]

Hallo,

it looks like libextract-gstreamer do not use nfo:frameRate. Any plans
to add it? 
There is a lot of files produced by Panasonic cameras
technically it is type = 'video/quicktime'; practically it is audio file
with really low video frame rate (= 0.2). So it is actually like mp3
with integrated image.
I need it to distinguish between videos (with fps = 30) and audios (fps
= 0.2).

Please add it :)

Regards,
Alexey