Ensure all panels take at least 300pt of width to ensure a minimum of
usability, readability of the panels and consistency of when the shell
folds itself.
With the new sidebar view in place, a new problem arose: we
need the current panel to be configured to update the title,
but cc_panel_list_add_sidebar_widget() changes the view while
activating the panel. That makes Settings crash.
Fix that by explicitly updating the headerbar widget, instead
of using notify::view of the panel list.
Only the date time panel used it, all other panels add their own shell using
cc_shell_embed_widget_in_header which was added after the date time panel was
written. Update the date time panel to use this method.
