Add AccordionPanel.setClosable(boolean)

[sveinnetnordic]

Describe your motivation

Having a message thread where one message always should be open.

AccordionPanel of Accordion should not close on click or enter. The panel should only close when an other panel is opened.

Describe the solution you'd like

Add AccordionPanel.setClosable(boolean)

Describe alternatives you've considered

Expert chat and me came up with this solution:
snippets:

vaadin-accordion-panel[opened] {
      pointer-events: none;
    }

.style("pointer-events: auto") //Re-enable pointer events disabled on AccordionPanel

c.element.executeJs("this.onclick = (ev)=>{ev.preventDefault(); ev.stopPropagation(); return false}")

From expert chat to close on keyboard Enter. Will flicker, but keyboard usage is less used

messageThreadAccordion.addOpenedChangeListener { 
            if (messageThreadAccordion.openedIndex.isEmpty) {
                messageThreadAccordion.open(openedIndex)
            }
            openedIndex = messageThreadAccordion.openedIndex.asInt
        }

Additional context

No response