The NonCheckableButton class was previously used for situations where a
*checkable* button was meant to not be checkable by the *user*, but only
through code. This was useful mostly as a styling tool (and to confuse
developers like me as to what it actually did).
The disadvantage is that such a button - a button that is actually
checkable (has the checkable attribute) but ignores the user - still
gets recognized by QAccessible as a checkable button, which has
accessibility issues when used like a normal button (see QTBUG-110737).
We can still get the styling effect on *actually* uncheckable buttons by
giving them a class, so this widget should not be necessary.
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