feat(ui5-li-notification*): full-fill accessibility spec

[ilhan007]

We identified gaps in the accessibility implementation, that this change aims to address as follows:

NotificationListItem

• More Button: added aria-label="More"
• Close Button: added aria-label="Close"

NotificationListGroupItem

• Invisible text reading: added "read" or "unread", based on the "read" property.
• Root: added aria-expanded="true"/"false", based on the "collapsed" property
• More Button: added aria-label="More"
• Close Button: added aria-label="Close All"
• Toggle Button added title="Collapse Group" ("Expand Group") and aria-label="Collapse Group" ("Expand Group") - based on the "collapsed" property

NotificationListGroupItem

• new property "read" (previously available in the NotificationListItem only)

Note: I didn't set aria-setsize and aria-posinset, as it's not implemented for none of the list items. And, if needed, this will be implemented in the List/ListItemBase.

[ilhan007]

Hello @SAP/ui5-webcomponents-topic-rd can you review this change?