Adds the class denoted by @qname to @line.
You may check if a line has @qname by calling [method@GutterLines.has_qclass].
You can remove @qname by calling [method@GutterLines.remove_qclass].
a line number starting from zero
a class name as a #GQuark
See Implementation
Adds the class denoted by @qname to @line.
You may check if a line has @qname by calling [method@GutterLines.has_qclass].
You can remove @qname by calling [method@GutterLines.remove_qclass].