UI improvements

[BrendanParmer]

This PR begins the revamp of NodeToPython's UI, in preparation for supporting more node tree types and exporting multiple node groups at once.

Changes

Features

• Node groups are now added to a UI list according to their type. This replaces selecting node groups at export-time, and will enable exporting multiple node groups in the near future
• Exports are now done with a single button, whose label/icon changes dynamically with the export mode, number of node groups, etc.
• New fields for exporting shader node groups, world nodes, light nodes, and line style nodes.
• The options panel has been cleaned up, grouping similar options underneath sub-panels
• Some option names and tooltips have been reworked to be more descriptive

Refactor

• Class and property registration has been revamped.
  • Each module now defines an optional variable classes: list[type] to be registered and unregistered in the base __init__.py
  • Each module now defines optional functions register_props() and unregister_props() for registering global variables to the scene.
• Classes and scene properties have been renamed to align closer to Blender's naming conventions

Other

• Removes vestigial NodeToPythonMenu class, which wasn't actually used
• Fix up some warnings, incorrect type hints, unused imports

Issues

Completed

• [suggestion] Add "copy to clipboard" button #131

In Progress

• Native support for exporting a collection of node groups #122
  • UI added for selecting multiple node groups
• UI added for selecting new node group types
  • Add World Nodes #172
  • Add Line Style Nodes #173
  • Add Light Nodes #174

TODO

• Exporting multiple node groups is currently unsupported. There's some design questions I still need to work out before the release. If it can't be done in time for 4.0 the UI should be updated to make it a bit clearer that only exporting one group is supported, I'm hoping not to need to do this though.

[BrendanParmer]

I imagine a lot of these classes could probably inherit from a common base class, might not have time for this release though

[BrendanParmer]

NodeGroupType and NodeGroupGatherer probably aren't appropriately placed