More dynamic attributes for inputs

In a recent thread I suggested including Type in the list of Dynamic Attributes for input. I think it would also be useful to have Placeholder added. In fact, there is probably a case for making any attribute dynamic and available in the UI.

This is obviously not an important request; it is straightforward to add this to the code manually. However, having it included in the UI confirms that it’s an option and will work - and saves a few keystrokes.