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/libs/utils/basetreeview.h

@@ -70,6 +70,7 @@ public:
 
     void showProgressIndicator();
     void hideProgressIndicator();
+    void resizeColumns();
 
 signals:
     void aboutToShow();