Further Qt6 fixes, drop legacy Anki support, slight refactoring

[glutanimate]

• Fix a couple of remaining unscoped Qt enums
• Drop legacy hooks and imports
• Refactor init to improve static type checking

Taken together with previous changes, the minimum supported Anki version is now 2.1.54, so we should create a new branch on uploading to AnkiWeb.

[ManabBala]

@glutanimate is this version live on anki-addon?