Clang: Display also child diagnostics in tooltips

...by introducing a custom tooltip widget for diagnostics.

Locations and fixits of child diagnostics are presented as clickable
links, leading to that position in the editor or to the execution of
that fix it.

Change-Id: I83e801e22d0421dd29275e333e5dd91587885cf1
Reviewed-by: Alessandro Portale <alessandro.portale@theqtcompany.com>

src/libs/utils/tooltip/tooltip.h

@@ -78,6 +78,7 @@ public:
                      const QString &helpId = QString(), const QRect &rect = QRect());
     static void move(const QPoint &pos, QWidget *w);
     static void hide();
+    static void hideImmediately();
     static bool isVisible();
 
     static QPoint offsetFromPosition();