This PR enables TreeTables to be used via keyboard and screen readers for improved accessibility.
It converts the expand/collapse toggle from a span-tag into a button tag. Adjusted css rules ensure that there is no visible difference. It adds two new attributes labelExpand and labelCollapse with appropriate defaults ("Expand" and "Collapse") in order to indicate the current state to screen readers.
added keyboard and screenreader support to TreeTable
Thank you, I'll merge but replace button with anchor since we use anchors mostly for accessibility.