Tooltip Accessibility #1012

Based on https://www.w3.org/TR/wai-aria-practices/#tooltip it should

• never receives focus. Focus stays on the textbox.
• be shown via keyboard focus or by the onmouse event.
• be hidden by removing focus from the text box or by moving the mouse off the textbox.
• be hidden by pressing the Escape key (bootstrap currently doesn't do this)

Also,

• The element that serves as the tooltip container has role tooltip.
• The element that triggers the tooltip references the tooltip element with aria-describedby. (not sure if this is realistic or possible, we probably depend on the developer using the component to add this, but it would be nice if it happened automatically while keeping the current API for tooltips which allows them to be disjoint from their trigger)