Widget movement enhancements.

* python-django.el (python-django-mode-map): Use widget-forward
and widget-backward as forward-char, backward-char replacements.
(python-django-ui-tree-dir-widget-expand): Expand directory names
correctly.
(python-django-ui-widget-move),
(python-django-ui-move-to-closest-icon): Enhance behavior when
using the text based tree.