TreeModel: API for setting header tool tips

Change-Id: Ieea98d244d689778db868604d981badcd166a7b5
Reviewed-by: hjk <hjk@theqtcompany.com>

src/libs/utils/treemodel.h

@@ -257,6 +257,7 @@ public:
     ~TreeModel();
 
     void setHeader(const QStringList &displays);
+    void setHeaderToolTip(const QStringList &tips);
     void clear();
 
     TreeItem *rootItem() const;
@@ -301,6 +302,7 @@ private:
 
     TreeItem *m_root; // Owned.
     QStringList m_header;
+    QStringList m_headerToolTip;
     int m_columnCount;
 };