Bug 1017294 - Add fastforward/rewind to music widget

Does the ``<a>`` element receive the contextmenu event but ``<button>`` element? if so, that should be a platform issue and let's have a comment to describe why we use the ``<a>`` instead of ``<button>``.

And if possible, I prefer to use ``<div>``, ``<a>`` has some default behaviours which might cause unpredictable issues.