Refactor particle_spawner system

[mnmaita]

• Removed some redundant code from a system that was setting both the color of the sprite and the index of the texture atlas, which is no longer necessary after SpriteSheetBundle was deprecated. TextureAtlas and Sprite components are now independent so we can update Sprite on one system and the TextureAtlas index on the other.
• Refactored particle_spawner code to reduce duplication. It should be now clearer that the difference between ParticleSpace lies in how the entity hierarchy is built. I feel like this could be simplified a bit more, but I didn't bother fighting against multiple mut borrow issues of Commands right now.
• Fixed a small typo in a comment.

[mnmaita]

@abnormalbrain @theanti9 @Bytekeeper you might want to take a look at this before releasing the next version of the crate. I'm mainly concerned about the changes I introduced in my first commit, which will remove some redundant work in systems now that Sprite and TextureAtlas are decoupled in bevy. Feel free to cherry pick just that for releasing, and then review or close this refactor if it feels irrelevant. Thanks!

[mnmaita]

Also, IMHO a good next step after this would be entirely removing ParticleTexture and having an independent way of adding the Texture Atlas to match Bevy current API.

[theanti9]

@mnmaita

Always in favor of PR's that remove more code than they add. This looks great.