We used to call QTextCharFormat::setToolTip from the ExtraSelection to
install the diagnostic tooltip. Since this allows to set only text
tooltips and we would like to introduce a custom tooltip widget for
diagnostics, make use of CppHoverHandler, which is more flexible.

Change-Id: Ia1b2c3c50810596ce4a3a025002e6e4efd8789db
Reviewed-by: Alessandro Portale <alessandro.portale@theqtcompany.com>