Checkboxradio: Don't re-evaluate text labels as HTML

Checkboxradio: Don't re-evaluate text labels as HTML

If you generate a Checkboxradio from a checkbox/radio with a label that
contains encoded HTML, e.g. `<em>test</em>` this will work fine
at first. If, however a refresh is triggered on that instance (explicitly or
e.g. by turning it into a `Controlgroup`), the previously escaped HTML will
now be evaluated.

If the label was created based on some user input, this could lead to
unexpected code execution even though the initial output was escaped.

Fixes gh-2101
Closes gh-2102