rewrite local media scanner (#25)

* spike new scanner

* add existing items to new scanner

* add collection refresh actions

* add tv show metadata and posters

* update metadata and posters when nfo/poster files are updated

* add "remove" action, test for all supported file extensions

* update statistics when primary video file is updated

* reflect that collections are "sourced" from nfo

* implement most scanning actions

* cleanup

* fix startup

* cross-platform scanner tests

ErsatzTV.Core/Metadata/LocalFileSystem.cs

@@ -10,6 +10,9 @@ namespace ErsatzTV.Core.Metadata
 {
     public class LocalFileSystem : ILocalFileSystem
     {
+        public DateTime GetLastWriteTime(string path) =>
+            Try(File.GetLastWriteTimeUtc(path)).IfFail(() => DateTime.MinValue);
+
         public bool IsMediaSourceAccessible(LocalMediaSource localMediaSource) =>
             Directory.Exists(localMediaSource.Folder);