Improve explosives.

[kindaro]

There are some features that the standardized explosives #113 do not yet have.

• Chain reaction: an explosion triggers all the other explosives in the range.
• Trigger delay: some explosives do not explode immediately upon triggering, but with a slight delay.

Particularly, mines #99 should have these features. Possibly also grenades #98.

[olefasting]

I have added both of these to the TriggeredEffect variant of WeaponEffectKind, which should typically be used for armed explosives. I have also added is_explosion to the CircleCollider variant of WeaponEffectKind, which should be used for explosions (triggered by the aforementioned TriggeredEffect). This will set off any TriggeredEffect instances that have Explosion as a valid trigger criteria, if they are within the radius of the explosion's collider.
I have also added Projectile as a trigger criteria, so that you can shoot explosives to trigger them....