[Tracker] Prune w/blroots

[so-rose]

To keep sane, I'm making a library that simplifies and modularizes many of the repetitive things I've done over the past 60,000-or-so lines of Blender addon code (so far the library is only available at https://git.sofus.io/so-rose/blroots). It will soon have an initial alpha release, and the plan is to let bpy_jupyter be the first "real" test-bunny for this.

Why is this good for bpy_jupyter? In short, extreme simplifications for more expressive logic. In particular, all the non-special things about bpy_jupyter as a Blender extension are planned to be mindfully pruned.

This Issue tracks the implementation of this refactor, which will extend over the following specific changes:

• bpy_jupyter.utils.*: Removed entirely; everything is integrated in blroots.
• bpy_jupyter.registration: Removed; integrated in blroots.
• bpy_jupyter.services.init_settings: Removed; integrated in blroots.
• bpy_jupyter.contracts.*: Contracted down to a single file containing OperatorType and PanelType; the rest is integrated in blroots.
• bpy_jupyter.__init__: Contracted greatly, due to sensible standardization of registration pipeline.
• bpy_jupyter.preferences: The property descriptors from blroots.bl_prop contract this greatly.

What's left? Ideally, "thin logic" around what actually matters: The service and UI that helps one run a Jupyter notebook kernel in Blender.