Accessibility issues with FilteredSelectMultiple widget

[Sarah Boyce]

1. "Available {name}" and "Chosen {name}" are h2 elements when this doesn't make sense for the html structure (makes them too important) (See headers video file)~ refs #34619
2. The select boxes have no labels, they should be labelled by "Available {name}" and "Chosen {name}" (see "Other issues" video file)~ refs #34619
3. "Choose all" is not related to it's tooltip help text (see "Other issues" video file)
4. The help text "Hold down “Control”, or “Command” on a Mac, to select more than one." should be associated to the select box(es) (see "Other issues" video file)

Possibly the whole thing should be a fieldset with a legend

[Sarah Boyce]

1 and 2 is a duplicate of #34619, going to add the other two points onto that ticket