Add ability to define smoothing groups

[neoascetic]

First of all, thank you for your outstanding work.

Is your feature request related to a problem? Please describe.
It would be nice to control make smoothness of some parts of edited mesh.

Describe the solution you'd like
Add a new tool for faces selection mode, where you can define up to N smoothing groups. Similar to Unity's ProBuilder feature, perhaps.

Additional context
I guess SurfaceTool is capable of doing this via add_smooth_group method, but I am not too familiar with ply's code and not sure where exactly to add the changes. Perhaps here?