mediawiki.util: Fix roundtripping of tooltip in portlet links

In 558985f72a ctrl-option- for Chrome on Mac was added, but it
didn't add "option-" to the regex. Since then the tooltip in
Chrome on Mac (and later when we started referring to option
instead of alt on all Mac browsers) was never detected by this
regex, sometimes resulting in double tooltips (when adding) or
outdated tooltips (when updating).

Now we always look for an accesskey hint in the tooltip and
strip it it's there.

Also fixed a bug where an undefined error can occur if accesskey
is given but tooltip not (the method unconditinally appended
text to the tooltip varible which might be undefined causing
a tooltip like "undefined[a]").

Change-Id: I0bde1a228983c58b20cad0c09a8e5efe8225ea23