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) {