Introduce node properties

[juzzlin]

Just had an idea that we could make it possible to add "properties" to nodes, for example hyperlinks and images.

Not yet sure how this could be shown in the UI, however.

"Add property handle" => "Select property type" => "Set property value/content". The node would then show a small property icon indicating that it has attached properties or even show them inline if only few.

[juzzlin]

Could be implemented as special nodes like suggested here: https://github.com/juzzlin/Heimer/issues/123