Refactor and Break up angle_frame_connector Module

[gbroques]

The angle_frame_connector.py module is getting quite large at around 500 lines.

It's also used independently from the frame as a stand-alone module from the Angle Frame Connector macro to generate corners for 3d printing.

The angle frame connector has one core component -- the "tri bracket" (better name TBD). We make the tri-bracket three times, and apply the appropriate rotations-and-translations to make the full corner -- taking advantage of the symmetries involved.

Top corners have an additional component -- the "axis side mount" or "tab".

Additionally, the angle frame connector has different features. Namely the ability to have a set-screw mechanism, and the option to apply filleting.

Each one of these components and features could be their own private module in a separate angle_frame_connector package which only exposes a AngleFrameConnector.make interface.

This could greatly help manage the complexity of this part.