Suggestion: Load bluebeard.bty from TC mod folders

[GooberRF]

Similar to #234 (regarding Bink video files), allowing mod authors to customize bluebeard.bty in their TC mods would enable some capabilities in mod creation that are either unavailable or "hackable" right now.

Most relevant with bluebeard.bty for this purpose is the "+Looping Sound" option set in the Sound Information section for a given filename. This option is required for designers to use custom sound files as looping sounds (e.g. ambient sound objects, mover looping sounds, etc.)

In TC mods, this can be "hacked" now by replacing a file already referenced in bluebeard.bty, but this approach isn't very flexible and has downsides.