Unified Search Widget information tooltip now is auto-hidden when the
control lost the focus (inside Thunderbird or when changing to another
app) and auto-showed when focused again (when the text field for search
keywords gets the focus). With this, the information is lost when changing
the focus but thats minor opposite to the major problem that was the tooltip
appearing on top of every window and every application (almost on MS Windows)
while a filtering was being applied.
Fixes #17, finally the XUL Tooltip
element is still being used instead of look for a alternative and now
I re-consider that is not so important show that information all the time,
only when a filter is applied and focus on the widget.