CORE-5175 Fix Folder refresh handling.

Partially reverted Folder refresh handling changes from commit c0bd26dbb166a56ff391656f4cfe6eb5a3821d6a.
Restored DiskResourcesEventHandler as a DiskResourceRefreshEventHandler.
Renamed the DiskResourceViewToolbar.Presenter#refreshFolder method to onFolderRefresh to make it more clear that this method is only for handling refresh events, where doRefresh is only for firing the event or refreshing search results.
Updated documentation on these methods.