Reworked FFmpeg path discovery and always display to user

1) Reworked FFmpeg and FFprobe path discovery (CLI switch, Custom xml, system $PATH, UI update trigger).  Removed FFMpeg folder from Emby.Server.Implementations.  All path discovery now in MediaEncoder.

2) Always display FFmpeg path to user in Transcode page.

3) Allow user to remove a Custome FFmpeg path and return to using system $PATH (or --ffmpeg if available).

4) Remove unused code associated with 'prebuilt' FFmpeg.

5) Much improved logging during path discovery.

MediaBrowser.Model/Configuration/EncodingOptions.cs

@@ -8,7 +8,8 @@ namespace MediaBrowser.Model.Configuration
         public bool EnableThrottling { get; set; }
         public int ThrottleDelaySeconds { get; set; }
         public string HardwareAccelerationType { get; set; }
-        public string EncoderAppPath { get; set; }
+        public string EncoderAppPathCustom { get; set; } // FFmpeg path as set by the user via the UI
+        public string EncoderAppPath { get; set; } // The current FFmpeg path being used by the system
         public string VaapiDevice { get; set; }
         public int H264Crf { get; set; }
         public string H264Preset { get; set; }