Function toggleCheckbox

Set or rotate checkbox status with support for tri-state.

An initial 'indeterminate' state becomes 'checked' on the first call.

If the input element has the class 'wb-tristate' assigned, the sequence is:
'indeterminate' -> 'checked' -> 'unchecked' -> 'indeterminate' -> ...
Otherwise we toggle like
'checked' -> 'unchecked' -> 'checked' -> ...