Add a way to create a new QGIS line layer which can be used as a gate layer

[JuhoErvasti]

Add a function that returns a line layer which a valid GateLayer can be created from.

• Probably a new module is needed for this? It could be a stand-alone function though at least to start with
• This can be done as a separate issue/PR but we could add a style to the layer which indicates the directions that the gate counts by default