default frameskip to 1

no more missing psi attacks.
they're only up for 4 frames, and can be skipped entirely.
:(
add frameskip to advanced options menu

new strings added:
STR_MAXFRAMESKIP
STR_MAXFRAMESKIP_DESC
make explosions more prettyful

allow some of the explosions to start animating after others have finished
by reducing the minimum possible starting frame to -3.