Lots of misc stuff

- Bomb overlay is invisible during comeback timer, and flickers back in
when it's almost up
- Show comeback timer on HUD
- Feather is stronger & has more gravity
- Option to use SMK star alarm instead of overlapping music
- Using P_MobjDamage normally on players now defaults to normal spinout
instead of shell's instant stop
- Some general gametype case fixes (most notably, being able to spin
people out in Race using a mushroom)