ui: use ListBox.focus instead accessing the ListWalker object

Although ListBox.get_focus() is deprecated, using ListWalker.get_focus()
through ListBox.body.get_focus() is not.

However, the ListBox.focus property exists and makes it easy to access
the underlying focused widget ; so use it instead of dereferencing the
ListWalker object.

Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>

subiquitycore/ui/container.py

@@ -465,12 +465,11 @@ def render(self, size, focus=False):
 
             seen_focus = False
             height = height_before_focus = 0
-            focus_widget, focus_pos = lb.body.get_focus()
             # Scan through the rows calculating total height and the
             # height of the rows before the focus widget.
             for widget in lb.body:
                 rows = widget.rows((maxcol - 1,))
-                if widget is focus_widget:
+                if widget is lb.focus:
                     seen_focus = True
                 elif not seen_focus:
                     height_before_focus += rows