Add the given Simple Control UI element to this container. Once the
control is added it cannot be modified. Controls are renedered in the
same order they are added. Controls that are Containers and can add
elements as well, like Group or Repeat, must be added BEFORE other
controls can be added to them. Controls that need references, like Inputs
(with property paths), or Submits (with IDs) will be given an automatic
reference if none was set. This reference will be returned by this
method, in the form of an Array of Strings representing the reference: a
Path for Inputs, or a single String for Submits IDs.
ctrl - The Simple Control UI element to be added.
The Array of Strings representing the reference Property Path for
Input Controls, or with a single String for Submits IDs.