Add 'split vertical layout' similar to line splitting

While editing/refactoring plaintextgen vertical/indent layouts with property macro's, it becomes tedious to move around multiple lines. Those cannot be selected at once, and therefore the lines need to be moved individually. Moving the macro lines doesn't work well with the move up/down in child list commands either.

Therefore, I propose that we can split a vertical layout into two vertical layouts by an intention. This would make it possible to move the whole vertical layout, e.g. when refactoring some code into a function + function call.

This can get a bit gnarly when the parent's direct contents allows only one element, then it may need to be wrapped by another vertical element.

Start by locating the plaintextgen vertical/indent layout editor, the existing line-splitting intention, and the move up/down in child list commands. Trace how property macros and parent contents that allow only one element are handled; done means an intention can split one vertical layout into two movable vertical layouts, including the noted wrapping case.

Written by the indexing model from the issue text.