Replace tooltiptext with updated dynamic tooltip.

Bug 979479 added dynamic tooltips, including keyboard shortcuts, to
various buttons; this change needs to be reflected in the overridden
XUL.  Before updating, this manifested itself as tabs overlaid on each
other, and the error message "undefined entity" (referring to the old
tooltiptext).

See http://hg.mozilla.org/mozilla-central/rev/4dc06a48ed13.
Merge pull request #1 from ncalexan/update-after-bug-979479

Thank you!

* Replace tooltiptext with updated dynamic tooltip.