CORE-4927 More refactoring to use folder paths.

Added DiskResourceView#getFolderByPath so that lazy-loading logic in SelectFolderByIdLoadHandler and DiskResourcePresenterImpl can still find a parent root folder of a folder that hasn't been loaded yet, and so the "Empty Trash" action can properly reload the Trash folder.
Added a DiskResourceView#getParentFolder method for the DiskResourcesEventHandler, so that it doesn't have to parse a folder's path to find its parent.
CORE-4914 Prevent filtered folders from loading.

Added a Folder#isFilter check to FolderRpcProxy and FolderContentsRpcProxy so that the service is not called if the folder is a filtered folder.
Removed obsolete TreeBeforeCollapseHandlerImpl since folders will remotely load in GXT 3.0.6 before the BeforeExpandItemEvent is fired, so canceling the event will not prevent filtered folders from attempting to load.