When removing all contents from the NumericInput box in PDFViewer the
callback set the (empty) text again back in the box, triggering another
callback in a recursive, non-stopping fashion. Not setting the text back
in the box avoids the problem.
Each of these strings would previously rely on StringView's char const*
constructor overload, which would call __builtin_strlen on the string.
Since we now have operator ""sv, we can replace these with much simpler
versions. This opens the door to being able to remove
StringView(char const*).
No functional changes.
