Add API to set a custom widget as a ColumnView header
Description
In GTK3 there was the function TreeViewColumn.set_widget which could be used to set a custom widget as the header of a TreeViewColumn.
In GTK4 the replacement for TreeViewColumn is ColumnViewColumn which only provides the ability to set custom text as the header, not a custom widget. This is therefore a feature loss from GTK3, and seemingly rather an unnecessary one.
Would it possible to add this feature back (I can have a go), or is it a deliberate design decision because the feature would cause issues?
Example use-case
In Inkscape a custom header widget is used in the XML editor for two things: to use an icon as the header instead of text, and to make the icon clickable: