Bugfix: Allow simultaneously adding cards and browsing cards

[psii]

Fixes a bug where simultaneous adding new cards and having the card browser open for editing stops the auto-fill feature from working correctly.

edit.EditManager wrongly assumes that there is only one Editor instance. The window for adding cards and the browsing window each instantiate their own editor.

Other small changes include replacing some legacy GUI hooks, see

• https://addon-docs.ankiweb.net/#/hooks-and-filters?id=new-style-hooks
• https://github.com/ankitects/anki/blob/main/qt/tools/genhooks_gui.py

[jdlorimer]

Much appreciated.

I'm very likely to merge this once I've resolved the more critical issues and had some time to test the changes.