Support UE1 animation workflow

[cmbasnett]

People using this add-on for UE1 animations have to do a lot of manual "frame counting" and and physically type out the frame ranges for animations after combining all of their animations into one mega-animation. This workflow is horrible, of course, and could easily be automated.

For the PSA exporter, add an option to switch to "UE1 mode", where the sequences are automatically concatenated together, and write a sidecar file of all the UE1 #exec commands, like below (full sample file also attached):

#exec ANIM SEQUENCE ANIM=AKs74uAnim SEQ=Idle STARTFRAME=1 NUMFRAMES=240 RATE=60 //GROUP=Idle
#exec ANIM SEQUENCE ANIM=AKs74uAnim SEQ=TriggerPull STARTFRAME=241 NUMFRAMES=1 RATE=60 //GROUP=Firing
#exec ANIM SEQUENCE ANIM=AKs74uAnim SEQ=HipFire STARTFRAME=241 NUMFRAMES=9 RATE=60 //GROUP=Firing
#exec ANIM SEQUENCE ANIM=AKs74uAnim SEQ=HIPtoIS STARTFRAME=250 NUMFRAMES=14 RATE=60 //GROUP=Idle
#exec ANIM SEQUENCE ANIM=AKs74uAnim SEQ=ISIdle STARTFRAME=263 NUMFRAMES=240 RATE=60 //GROUP=Idle
#exec ANIM SEQUENCE ANIM=AKs74uAnim SEQ=ISFire STARTFRAME=503 NUMFRAMES=9 RATE=60 //GROUP=Firing

AKs74u.txt