Make Bullet projectile extensible

[michaeldgg2]

This PR makes Bullet projectile extensible, so mods can adjust and modify its behavior.

Proposed changes would be used in OpenE2140 mod to implement debris flying out of spot, where original projectile (i.e. the one fired by actor's weapon), hits the target.

Draft implementation of new FreezingBullet projectile (based on the new, extensible Bullet) can be seen in this commit: michaeldgg2/OpenE2140@5a37d20. This commit, however, contains only the use of this new projectile (not the entire implementation of the debris).

This is how the debris with FreezingBullet looks like. Heavy tank with three cannons shooting at the ground. Notice the small pieces of debris flying out of spots, where the bullets from the tank's cannons hit the ground.

Current version of this feature in OpenE2140 uses CreateEffect warhead as can be seen here:

https://github.com/OpenE2140/OpenE2140/blob/master/mods/e2140/content/core/weapons/debris.yaml#L14

This implementation has however two bugs:

1. there's one tick delay before CreateEffect warhead kicks in and creates SpriteEffect
2. sometimes the appearance of Bullet and SpriteEffect created by CreateEffectWarhead doesn't match

First issue is visible only on low FPS. The second is caused by both Bullet and CreateEffectWarhead picking different random sequence for rendering.

Both issues cannot be worked around and require changes in OpenRA (unless Bullet and CreateEffectWarhead are reimplemented from scratch).

[anvilvapre]

Seems to do what the title sais and cleanly.

Feels like bullet is, was already, very custom/hacky. Makes me wonder if it couldn't be cleaned up.

[anvilvapre]

Will likely also have quite a negative peformance overhad with many bullets flying around.

[michaeldgg2]

I did take inspiration from Actor.Renderables(), but I can quickly turn it into standard (non-iterator) method.

[abcdefg30]

Untested. Code changes lgtm those two nits aside.

[PunkPun]

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