Show Backwall Tooltips in Tiles

[Pagox]

In natural tiles, both the foreground element and the background element are displayed in the tooltip.

In build tiles on the other hand, only the foreground element is shown in the tooltip, but not the background substance.
This works only for airflow and mesh tiles.

It's always unclear whether there's anything behind a tile, so you have to destroy it.

Request:

1. Always show the backgroud element in the tooltip.
2. (Optional) The tool filter suggest, that you can choose the layer the duplicates should dig.
   - Tiles yes / Background no -> works
   - Tiles no / Background yes -> doesn't work
   Either it shouldn't be possible to select that combination, or it should be possible to do so.