log: properly handle iso8601 timestamps
The panel-plugin menu list and the config log menu list both depend on iso8601 timestamps to correctly display the timestamp in the item tooltip. However, a timezone was not specified, resulting in a time of "NULL" being displayed.
This change modifies common/xfce-notify-log to write out iso8601 timestamps directly using the is8601 g_date_time* functions. It then adds timezone localization support for the tooltips.
This change also fixes some memory leaks handling the dates in both the config menu and the panel plugin menu.