Unregister bogus event handlers when destroying a wbtooltip

It's hard to reproduce this but I hope it's obvious. It happens
to me when I try to use the "Add links" feature in my local
client, it fails (for an other reason) and shows the error in a
tooltip. Then I click "Add links" a second time. This destroys
the tooltip but leaves the two mouseenter/leave event handlers.
These handlers trigger type errors in the JS console because the
rest of the widget is gone.

Change-Id: Iad49bbcf78e05bd853ca9d791f5a81d14969413d
Merge "Unregister bogus event handlers when destroying a wbtooltip"