Adds back previous ParticleEffect API methods and minor convention chang...

Commit d1bc56aee0f5be6a14f838a16866df8b2b25487d has made some changes to ParticleEffect API and has been implemented in a way that, in my opinion, shouldn't have been merged. 

I have made some changes to bring back the previous API methods not to break backwards compatibility (plus some minor changes) but I believe the whole thing should be rethought. I may have missed the idea but I'm not sure what it tries to solve.

 There doesn't seem to be any documentation relating this convention either.
```
if (atlasPrefix.length() > 0) imageName = atlasPrefix + "/" + imageName;
```
Now if atlasPrefix equals "", it doesn't matter anymore, no need to validate