Enhanced keyboard navigation for Offcanvas.

Enhanced keyboard navigation for Offcanvas.

On open not the first element within the side panel, but the whole panel should receive focus.
If the first interactive element is focussed the user might miss some content above it, e.g. the panel heading.
Also added usage of handleKey() to the component to close the panel.
Moved the panel focussing outside of the transitionEnd listener because this was never called and it also works outside...