copy attributes for fx-fore with src

[JoernT]

When using fx-fore with the src attribute the original attributes of the fx-foreelement shold be present on the imported fx-foreelements after replacing the element.

There can be clashed if the imported element has the same attributes. In that the attribures on the importiing element should take precedence.

Values for the class attribute should be merged.

Adding this behavior would allow to pass params down to the imported fore element.

[JoernT]

done on branch and gets merged before next release.