Improvements to the particle editor

[ZehRhayader]

1. Ability to upload custom textures / sprites to the particle editor
2. General UI / UX improvements to the particle design flow
3. Way to edit the update method for advanced effects of the emitter
4. Easier access to the particle editor

[KilledByAPixel]

I just added drag and drop for custom textures and also added way to set the padding and size so it can be modified for the custom texture.

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