Add layer mask duplication operator

[BittieByte]

Initial version of the layer mask duplication operator. The feature is currently functional but incomplete.

Known issues:

• Duplicate creates a new decal object instead of reusing the original source.
• Duplicated mask is not inserted directly below the original.
• Some properties such as transform values, intensity, and modifiers are not yet copied.

CLOSES #153

[ucupumar]

Oh yeah, I don't realize that you can just create a new mask with all properties copied from the original mask. I thought duplicating a mask is similar to duplicating a layer, which actually copies the node manually. Okay, I'll test this later. Thanks for the PR! 👍