True padstacks and via stacks with differing geometries on different layers (lp:#1827233)

• Via tenting control
• Via padstack shape control per-layer
• PTH padstack shape control per-layer
• SMD pads on inner layers moved to subtask
• Filled via support moved to subtask
• Backdrill support moved to subtask

Original report created by Chris Elliott (sembazuru)

It would be great to have true padstacks to allow different geometries
across layers. The primary use for this would be to support proportional
THT padstacks where the inner layer annular rings are smaller than the
exterior annular rings, and maybe even a setting to not place an annular
ring on an internal layer that doesn't have an electrical connection.

This can be expanded to allow custom pad shapes on any layer, and different
custom pad shapes on different layers. This may be a benefit for any future
expansion towards mixed flex and rigid design support.

Original tags: pcbnew