BaseTreeView: Make resizeColumns() publicly accessible

Simplifies the only used code path through the model and a custom
signal.

Change-Id: I9bbab725cdd122b7261a8f14b101786e38e61e57
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/debugger/watchhandler.h

@@ -50,7 +50,6 @@ signals:
     void currentIndexRequested(const QModelIndex &idx);
     void itemIsExpanded(const QModelIndex &idx);
     void inameIsExpanded(const QString &iname);
-    void columnAdjustmentRequested();
     void updateStarted();
     void updateFinished();
 };