Change way themes are added

[thomasp85]

I've created a textboxGrob that allows for automatic wrapping of text etc. I'm trying to create an element_textbox as a subclass of element_text to allow people to use it for titles etc, but there are some problems in how themes are added that makes it impossible to subclass elements.

Specifically the class of the added elements are dropped if the element is already present, since add_theme is merely modifying the slots of the original element rather than using the added element as the base.

Will you be willing to consider a PR that flipped this behaviour so that add_theme would use the added element as base and modify the NULL slots of this?

[baptiste]

sounds similar to #1240

[hadley]

Yes, I would be willing to consider a PR.