Allow for a transparent view of the parts on the build plate in editor mode

[seanmcne]

Is your feature request related to a problem? Please describe.
It can be frustrating when trying to view parts and align them when there are voids in the middle, it would be great to enable a transparent view of the parts on the build plate (similar to what is rendered initially when slicing)

Note: I screenshot this just as the slicer was building the gcode, I had just enough time to capture it. I think it would be helpful to enable this view of the parts so I could move them around and adjust how much they sink into the plate or even just for arranging.

As compared to:

[neophyl]

Duplicate issue of several, but I think the earliest is #3678