Add Horde_Core_Block::refreshContent()

This method has been called via Horde_Application_Ajax_Handler since
at least Horde 5.1, so make it part of the official API going forward.
Allow specifying a method to call when autoRefreshing a block.
Detect autoRefresh() vs refreshContent() requests.

Needed so we can avoid outputing the JS an autocompleter for the
weather location on each AJAX refresh.
Refactor weather block to use HordeCore.