Suboptimal UI for floor plan properties

There are a few things about the floor plan properties that are wrong or misleading:

- The minimum region area is listed as being a floating-point number in meters squared. In actuality, it's an integer number of Recast cells.
- Because it's a number of cells, it seems pretty fishy for users to fiddle with this while "auto cell size" is enabled, because you don't even have any way to know what the cell size is, and the cell size will change per our auto cell size algorithm as the scene they are building changes. It would probably be better to hide it when "auto cell size" is enabled.
- Cell size should be labelled in its units (meters, not meters squared.)
- It seems a little surprising that cell height is configurable even when "auto cell size" is enabled, but I can't say it's wrong, just surprising.
- It's kind of strange that you can't decrease cell height or cell size below 0.1m, because the Recast defaults for those are 0.02m and 0.03m respectively.

Locate the floor plan properties UI and inspect how minimum region area, cell size, and cell height are displayed and constrained. Compare the controls with the listed Recast units, auto cell size behavior, and minimum values; done means the labels, visibility, and limits no longer mislead users.

Written by the indexing model from the issue text.