Add features to simplify positioning of existing and new decals

[passivestar]

Adds 2 things:

• A button to set the position of existing decal object to 3D cursor
• A checkbox to set a new decal position to 3D cursor when duplicating a decal

s.mp4

Note: Code duplication for getting texcoord is potentially a concern, but I found that it's used in many different places so it's better to refactor it separately in the future

[ucupumar]

Thanks for another PR! I'm merging this immediately 👍
Anyway, what's the concern about getting the texcoord?

[passivestar]

Anyway, what's the concern about getting the texcoord?

There's just a lot of code repetition, potentially something to refactor in the future

[ucupumar]

I see, yeah there should be a more common function to check if the passed entity is a mask or not