Fix #385191 by using the new progress reporting API.

* ui.py:
(GtkProgressBar): Simplified, we don't need to inherit from
_BaseProgressBar.
(GtkProgressBar.update): Show the widget.
(ProgressBarWindow.__init__): Simplified. Don't show
unconditionally.
(ProgressBarWindow.update): Show the widget.
(ProgressPanel.update): Show the widget.
(GtkUIFactory.__init__): Simplified.
(GtkUIFactory._progress_all_finished): 

* branchview/treeview.py:
(TreeView.__init__): Set the progress reporting widget.