Previously the only way to control the subtype was to remove the group
input or output and create it again. This commit adds a dropdown to
change an existing socket, for supported socket types.
Based on a patch by Angus Stanton: https://developer.blender.org/D15715
It was necessary to fix the UI code slightly; the layout's context
wasn't being used in calls to an operator's enum items callback.
Pull Request: https://projects.blender.org/blender/blender/pulls/105614
e7f395dd20