Move Quake 3 player export/import out of MD3 filter

[zturtleman]

Add File -> Export Player... option to allow exporting Quake 3 player model in any format or player model segments (head, upper, lower) in different formats. This likely needs a Import Player... option as well. Unless opening any of the segments while the others are present in the same directory prompts for loading as a Quake 3 player.

It would also be nice to add support for RTCW and ET's head/body player models. It could be used with Inter-Quake Export (IQE) even if RTCW/ET model formats aren't supported.

I've started the export portion but haven't made a GUI for it yet.

[zturtleman]

I was planning to add this by copying model content needed by head, upper, and lower and then using regular model export for each part. However copying frame animations (using MM3D's C++ Model class API) takes too long to be acceptable.

So I need to either find a faster way or give up on moving player export out of MD3 filter.