The template use in the inspector was not properly
disposing all widgets. gtk_widget_dispose_template
will only unparent widgets that have been named
as template children, so we need to make the toplevel
elements in the ui file named children, or manually
dispose them. This commit does the former.
Rename the DataList object to TreeData, in preparation
for adding a ListData object for list models. While
we are touching it, modernize it a bit (drop the Private
struct, use a layout manager, etc).
