Draft: A few linedef action tweaks

• Some improvements to crumbling platforms:
  • Updated the flashing so it looks a little less weird.
  • The respawn timer can now be set, instead of being fixed at 15 seconds. This is done by checking the Peg Midtexture flag and using the front X offset to change the respawn time, in tics.
    • This also works for linedef action 446, Make FOF Crumble, but since that normally uses the front X offset for the sector tag, checking the Peg Midtexture flag makes it use the linedef's tag for the sector tag.
• Some improvements to plane scroller actions:
  • Added actions 516-518, 526-528 and 536-538, which affect both floors and ceilings simultaneously.
  • Plane scroller linedefs without a tag will now affect their front sector.

To-do:

• Make crumbling platform flashing more consistent.