Make the button widget apply selectedClass correctly when the state is
addressed with setTitle instead of set.
Issue #9949, the button could not be rendered at all
* Guard the setField read in isSelected(), which dereferenced the result of
getTiddler() directly. A missing state tiddler threw a TypeError, and that
tiddler is normally only created by the first click
Issue #9950, every button of a set reported itself as selected
* Rewrite isSelected() as an if/else. The nested ternary mixed `||` with
`?:`, so the setTitle branch returned a fallback value instead of a
comparison
* Watch the setTitle state tiddler in refresh(), so the selected class
follows the state as it already does for set
Documentation and tests
* Document that setField takes preference over setIndex. That has always
been the behaviour, it was never written down
* Document that selectedClass and default apply to setTitle as well as to set
* Add wiki based test cases for the set, setTitle, setField and setIndex
selection paths. Six of the seven fail against v5.4.1, the one that passes
uses set, which was never broken
Fixes#9949Fixes#9950
