Externalize Explosion Graphics

Fixes the following bugs:
- ExplosionBState.cpp: big explosions spawn sprites in circle around
centre, not square
- Map.cpp: offset for explosion sprites from X1.PCK now correctly offset
by 64,32 not 64,64

This means that the layout of explosion sprites should appear more
circular (most visible on large explosions) and be correctly centred
(most visible on small explosions).

Defines the following config:

- item config contains a new explosion entry (or defaults to
STR_DEFAULT)
- new explosion entries contain config that detail animation for
different explosions

Adds following new config:

- STR_DEFAULT explosion entry in Xcom1Ruleset.rul
- Improved_Explosions.rul has animations