event.relatedTarget -> clicked 'a' on hide/hidden

Rather than point to the '.btn.dropdown-toggle' when hiding a '.dropdown-menu' event.relatedTarget for 'hide.bs.dropdown' and 'hidden.bs.dropdown' events now refers to the anchor ('ul.dropdown-menu li a') that was clicked (if any).

Otherwise the behaviour is the same as previous - ie the '.btn.dropdown-toggle' is passed to event.relatedTarget (as fixed in previous commit).

This change allows for much easier/cleaner implementation of <select> functionality and it's worth noting that event.target (& this in the event handler) still refer to the 'div.btn-group' surrounding the 'button.toggle-dropdown'.

Also functions as expected (my interpretation of expected anyway) when the 'ul.dropdown-menu' is not a child if the 'div.btn-group' (or sibbling of '.btn.dropdown-toggle) and data-target it used.