Expose opaque material property in `Materials Tab`

People often request a way to disable/hide blocks such that they do not render.

`Specular` set to a value of `-1` is currently the only way to perform this action without editing the code however this method is unofficial and has numerous issues with the most egregious being it makes Chunky extremely unstable. This method also produces an x-ray bug type effect; That being directly adjacent blocks to the "hidden" block do not have a surface rendered.

ie -

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Exposing the `Opaque` material property in the `Materials Tab` would give people access to a much more stable and predicable method of hiding blocks.

As noted, we would need to include a tooltip, or note, indicating that Chunks need to be reloaded in order for these changes to apply.