This implements Box Trim as a boolean based trimming tool in sculpt
mode. This is the intended way to remove parts of the sculpt instead of
using box mask and mask slice. It also creates new face sets for the new
faces created after the boolean operation.
The code for the tool itself can be considered (almost) ready, but it is
using the code from the boolean modifier to generate the new mesh. This
makes it slow and it has a lot of precission errors when calculating the
intersections. When the new boolean code of T67744 is ready, the tool
should work by just replacing the applyBool() function with the new one.
If the new boolean code can skip the mesh->bmesh conversion, it would be
much better.