mediawiki.util: Don't hardcode selectors in updateTooltipAccessKeys if possible

mediawiki.util: Don't hardcode selectors in updateTooltipAccessKeys if possible

If we're running on a browser where we can do this efficiently, just
find all elements that have accesskeys using document.querySelectorAll.
We can't use jQuery's polyfill for the selector since looping over all
elements on page load might be too slow.

Otherwise (IE<=7, Firefox<=3), just as it was done previously, go
through some elements likely to have accesskeys and update only them.
Unfortunately this will not fully work for custom skins with different
HTML structures, but you can't have everything. Also added 'button' to
the selector used in this case.

Bug: 58255
Change-Id: I3306ac4b2d32a5f0f6567e6aab176ecd6682062f