Reset modified flag on undo.

src/com/jpexs/decompiler/flash/gui/MainPanel.java

@@ -5961,6 +5961,11 @@ public final class MainPanel extends JPanel implements TreeSelectionListener, Se
 
                 }
 
+                @Override
+                public boolean isModified() {
+                    return false;
+                }                                
+
                 @Override
                 public ReadOnlyTagList getTags() {
                     if (cachedTags == null) {

src/com/jpexs/decompiler/flash/gui/PreviewPanel.java

@@ -2778,6 +2778,11 @@ public class PreviewPanel extends JPersistentSplitPane implements TagEditorPanel
 
             }
 
+            @Override
+            public boolean isModified() {
+                return false;
+            }
+                        
             @Override
             public ReadOnlyTagList getTags() {
                 if (cachedTags == null) {

src/com/jpexs/decompiler/flash/gui/TimelinedMaker.java

@@ -305,6 +305,11 @@ public class TimelinedMaker {
             public void setModified(boolean value) {
             }
 
+            @Override
+            public boolean isModified() {
+                return false;
+            }                        
+
             @Override
             public ReadOnlyTagList getTags() {
                 return ReadOnlyTagList.EMPTY;