color: labeled text should be passed to ui.write() as ui.labeled
Some implementations of ui.label() (HTML versions in particular) must escape
the provided text and then markup the text with their tags. When this marked
up text is then passed to ui.write(), we must label the text as 'ui.labeled'
so the implementation knows not to escape it a second time (exposing the initial
markup).
This required the addition of a 'ui.plain' label for text that is purposefully
not marked up.
I was a little pedantic here, passing even ' ' strings to ui.label() when it
would be included with other labeled text in a ui.write() call. But it seemed
appropriate to lean to the side of caution.