[3.x] Improve Class display in Create dialog

[illlustr]

Backport #100729

• Button to open Script for custom class
• URI to Open Doc for native class

[KoBeWi]

The code looks quite different from the one on master. There are some questionable bits, but it's not wrong.
Be sure to test it though, because I didn't.

[KoBeWi]

The commits need to be squashed.

[lawnjelly]

Thanks!