Feature: Insert form elements add specific position

[idkCpp]

Let's say you have a login form. It contains a login name, a password and a submit button. Then you want to create a register form with an additional confirm password field.

With this you can just inherit from login and then add the second password field in the right place.

I just knocked this out. Comments are very welcome. This may very well be ABI-compatible with the current version (but I don't know much about this)