[Feature] Optionally play GEARUP.LFD

[NightFright2k19]

It would be nice if TFE automatically played the "Gear-Up" cutscene (gearup.lfd) from the DF demo if placed in the LFD subdir of the DF game dir. It's supposed to play as a part of the intro sequence, just before the Dark Forces logo appears.

I am not sure if this file is installed in the final DF release as well and just isn't used, but vanilla behavior could be maintained by creating a menu toggle for it (something like "Play Gear Up Intro (if installed)") which is deactivated by default.