Add easy way to add progress indicator overlay to a widget

Adds a way to 'attach' a progress indicator to a parent, so it
automatically keeps itself centered on the parent.
Also adds shortcut methods to BaseTreeView.

Change-Id: I962a9300d89ef5d1e03298d670e0cee781d3a4e5
Reviewed-by: hjk <hjk@theqtcompany.com>

src/libs/utils/basetreeview.h

@@ -59,6 +59,9 @@ public:
     virtual void rowClicked(const QModelIndex &) {}
     void mousePressEvent(QMouseEvent *ev);
 
+    void showProgressIndicator();
+    void hideProgressIndicator();
+
 public slots:
     void setAlternatingRowColorsHelper(bool on) { setAlternatingRowColors(on); }