Once upon a time there was the idea that WebODF could be also used with just a DOM, without any layouting done (e.g. in Node.js). I am not yet willing to give up on that idea, so given that `getContentRect` is just used by `gui.Caret` for now, could this method stay there for now? Or perhaps there should be a separate GuiStepUtils class (hm, could be a namespace acutally, given it is a stateless set of helper methods right now).