Add jquery.accessKeyLabel javascript module

Extracted the accessKey logic from mediawiki.util and moved it to it's
own module. mediawiki.util now depends on the new module,
moved the jquery.client dependency from mediawiki.util to the new
module.

Now the accesskey is also set, when no placeholder like '[x]' is set on
the title (bug 48505).
Also the label for a input or textarea (and some other elements) gets
updated (bug 63849).

Also implements support for the browsers accessKeyLabel function (bug
37239), which will give a localised accessKey string, if supported (at
the moment FireFox 8+ only)

Bug: 37239
Bug: 48505
Bug: 63849
Change-Id: I211593629ca23540dcd5a93b11834155f4f04883
Merge "Add jquery.accessKeyLabel javascript module"