Define storage roots, external GUIDs, creation.

Allow storage backends to publish multiple roots into the UI, which
are defined by a directory GUID, type, and label details.  Update
external provider to surface a primary external storage root, and
switch to burning file path into the returned GUIDs so they remain
durable.

Added insert, update, and delete support to external provider. Adds
file extensions to display names when needed to match MIME type.

Add flags for searching and deletion, and extras for Cursor
pagination. Add directory creation dialog to UI. Opening a document
always gives write access.

Change-Id: I9bea1aa0dcde909a5ab86aefeece7451ab920cf1