Add type check on track to prevent PLI for non-video tracks.

[mondain]

While looking at where to handle incoming PLI messages, I made the following minor changes:

• Fix typo in rtcpnackrsponder.hpp
• Remove irrelevant comment in rtcpreceivingsession.cpp
• Modify requestKeyframe method to prevent PLI on non-video tracks

[mondain]

@paullouisageneau is there a method or callback for a streaming application to catch PLI or FIR coming from the end-point to which they are connected, so that the application can provide a proper response; such as restarting their video encoder?

[paullouisageneau]

Thank you for the changes!

There is no callback to catch a PLI, because it would be part of a RtcpSendingSession which is not present in the library for now, meaning the sender application has to handle RTCP messages.