Allow copying/cloning of elements

[jegelstaff]

Instead of setting all properties for an element, have an option where you can choose any existing element is the one that this one clones/copies. All the settings of the parent will be used for the child.

Ideally, this is not a one time copy, but a reference back to the source. Perhaps bidirectional? The dependent element could simply have a property that references the parent element id, and the class can be made smart enough to retrieve a different record from the DB when getting this element.

[jegelstaff]

This way, you can edit any one of the connected elements, the parent gets the changes, and all the child elements get the propagated changes, because they are simply read from the parent.

There is some work to do to figure out if absolutely every element property is inherited, or if there are some that are explicit on the child.

Some work towards this here: #276