Don't expect BaseItem to be a movie/video file.

This fix is mainly so I can mass-add series _and_ movie entries using a
`IMultiItemResolver` without having to resort to complicated logic
using _both_ a `IItemResolver` and a `IMultiItemResolver` by splitting
up what gets added where.

I've also added three new interface methods to the `IDirectoryService`,
one of which is used in the modified
`ResolverHelper.SetInitialItemValues(…)` to get the file system entry
info for the item regardless of which type the file system entry is.

In my local testing so far I haven't found any issues introduced
by this change.

Emby.Server.Implementations/Library/ResolverHelper.cs

@@ -39,7 +39,7 @@ namespace Emby.Server.Implementations.Library
                 item.GetParents().Any(i => i.IsLocked);
 
             // Make sure DateCreated and DateModified have values
-            var fileInfo = directoryService.GetFile(item.Path);
+            var fileInfo = directoryService.GetFileSystemEntry(item.Path);
             if (fileInfo is null)
             {
                 return false;