TEMPLATE: Allow user-defined template source.

[JakobVogelsang]

Is your feature request related to a problem? Please describe. As a specification engineer, I want to be able to import LNodeType elements from other sources. I need this because I have a base SSD file where all my type definitions are being made, and I have separate SCL files where I have my types.

Describe the solution you'd like When I want to add a LNodeType in an existing project in the TEMPLATE editor I would like to have yet another option to import it from a file.

Suggestions for a specific user interaction Could be a plugin setting template.ssd or other file