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.