list_browser ->change_selection: action={set,toggle,extend,clear,cancel},
context=[dict_item|chain]Hook in selection management. This method is invoked from
list_browser->event
and may be used to keep track of selection changes in the list_browser
object. If action is set, the selection will
be cleared and then set to the argument dict_item
object. Action toggle will toggle the status of the
argument dict_item, while clear will empty the selection.
Redefinition of this method may be used to keep track of the
selection. See also list_browser->select_message
and list_browser->open_message.
