Add read-only line editor in the substation plugin

[JakobVogelsang]

As a user of OpenSCD I want to see Line element in the SCL in the substation plugin

Requirements

• make use of action-pnae
• title is "Line.name — Line.desc (Line>Voltage.innerText+Line>Voltage.multiplier)"
• show in all possible parents that is Process, SCL
• show all possible children, that is: ConductingEquipment, GeneralEquipment
• show all hiden children with showfunction=true that is Function, LNode

[pascalwilbrink]

According to the XSD, the Line element is a sibling of the Substation element. Should this really be in the substation editor, or should this become a new plugin? (Same with Process)