Support the gtk-doc action syntax
GTK4 allows adding widget-related actions to the documentation with the newly defined syntax:
<class_name> '|' <action_name> ':'
This means g-ir-scanner needs to detect this new format, to avoid emitting unnecessary warnings.
Currently, we don't do anything with the actions; in the future we might want to add them to the documentation in the GIR, but for that we'd need a new element.
See also: gtk-doc!30