fix(event-chart): Added aria-label to rendered event bubble.

When a role of button is applied an aria-label or some sort of label is required. For the event-bubble button it is save to assume that it will open the overlay (or at least select the element). This is controlled by the internal logic of the event-chart.

libs/barista-components/event-chart/src/event-chart.html

@@ -103,6 +103,7 @@
           "
           [class.dt-event-chart-event-selected]="_isSelected(renderEvent)"
           role="button"
+          aria-label="Select this event to open the corresponding details panel"
           tabindex="0"
           (click)="_eventSelected($event, renderEvent)"
           (keydown)="_handleEventKeyDown($event, renderEvent)"