Add placeholder fill feature

[someaddons]

Changes proposed in this pull request:

• Adds a new feature to the scan tool, which automatically fills the top area of a schematic with placeholders to have the schematic integrated much better into the environment. E.g. it won't cut out huge rectangles when placed into a mountainside
• Allows players with a scan tool to pass through light placeholder blocks
• Made light placeholders semi transparent

Review please

[uecasm]

Screenshot?

I'm not sure how useful a placeholder fill actually is. Typically in level 1 you don't want to use placeholders, and in higher levels you need to carefully set specific blocks to air even when you want the majority to be placeholders.

Being able to pass through placeholders would be nice though. Currently I use a two-step replace, where I swap the air to a unique block and then swap placeholders to air, then do the edits and then reverse this. Occasionally this messes up other blocks (like converting grass/paths to dirt, or changing fence attachments) though.

[someaddons]

Like for sure it is not perfect, thats also not the intention, but it'll make it easier to properly surrouned esp the air corners with placeholders to avoid buidings taking huge rectangle chunks out of the terrain

[someaddons]

[uecasm]

You might also want to write some usage docs for the changelog/wiki into the description. I don't really see how the numbers in the first screenshot would produce the result in the second.

[someaddons]

Usage doc is in tooltips on all the elements added to explain those