Simplifies and improves code. Removes some duplication

src/main/java/net/knarcraft/ffmpegconverter/streams/AbstractStream.java

@@ -7,6 +7,7 @@ public abstract class AbstractStream implements StreamObject {
     int absoluteIndex;
     int relativeIndex;
     String codecName;
+    String language;
 
     @Override
     public String getCodecName() {
@@ -22,4 +23,10 @@ public abstract class AbstractStream implements StreamObject {
     public int getRelativeIndex() {
         return this.relativeIndex;
     }
+
+    @Override
+    public String getLanguage() {
+        return this.language;
+    }
+
 }

src/main/java/net/knarcraft/ffmpegconverter/streams/AudioStream.java

@@ -4,7 +4,6 @@ package net.knarcraft.ffmpegconverter.streams;
  * This class represents an ffmpeg audio stream
  */
 public class AudioStream extends AbstractStream implements StreamObject {
-    private final String language;
     private final int channels;
     private final String title;
 
@@ -27,15 +26,6 @@ public class AudioStream extends AbstractStream implements StreamObject {
         this.channels = channels;
     }
 
-    /**
-     * Gets the language of the audio stream
-     *
-     * @return <p>The language of the audio stream.</p>
-     */
-    public String getLanguage() {
-        return this.language;
-    }
-
     /**
      * Gets the number of channels for the audio stream
      *

src/main/java/net/knarcraft/ffmpegconverter/streams/StreamObject.java

@@ -23,4 +23,11 @@ public interface StreamObject {
      */
     int getRelativeIndex();
 
+    /**
+     * Gets the language of the audio stream
+     *
+     * @return <p>The language of the audio stream.</p>
+     */
+    String getLanguage();
+
 }

src/main/java/net/knarcraft/ffmpegconverter/streams/SubtitleStream.java

@@ -4,7 +4,6 @@ package net.knarcraft.ffmpegconverter.streams;
  * An object representation of a subtitle stream in a media file
  */
 public class SubtitleStream extends AbstractStream implements StreamObject {
-    final private String language;
     final private String title;
     final private String file;
     final private boolean isFullSubtitle;
@@ -32,15 +31,6 @@ public class SubtitleStream extends AbstractStream implements StreamObject {
         this.file = file;
     }
 
-    /**
-     * Gets the language of the subtitle stream
-     *
-     * @return <p>The language of the subtitle stream.</p>
-     */
-    public String getLanguage() {
-        return this.language;
-    }
-
     /**
      * Gets the title of the subtitle stream
      *