multi-select support / edit multiple brushes along common shared planes

[radiatoryang]

I don't know how possible / complicated this is, but it'd be a huge help:

Let the user select / edit / adjust multiple brushes as the same time.

In TrenchBroom, this workflow is possible:

• user selects multiple brushes
• when adjusting a brush face, it also adjusts any coplanar face (a face with the same plane) in a selected brush
  (see GIF below)

• faces do NOT need to be flush / adjacent / connecting, they just need to share the same plane
• this is a crucial feature in TrenchBroom, essential for any brush based workflow
• given floating point imprecision etc. there should probably be some tolerance for detecting similar-ish planes
  • default tolerance: if a face's plane is within ~0.1 meters of the plane origin and ~1 degree of plane normal, treat it as coplanar

current state in Chisel:

• selecting multiple brushes = no gizmos or handles in 3D view
• must manually select each brush individually, and resize individually