Modularize and Doxygen decklist.cpp (#6099)

common/abstract_deck_list_node.cpp

@@ -0,0 +1,24 @@
+#include "abstract_deck_list_node.h"
+
+#include "inner_deck_list_node.h"
+
+AbstractDecklistNode::AbstractDecklistNode(InnerDecklistNode *_parent, int position)
+    : parent(_parent), sortMethod(Default)
+{
+    if (parent) {
+        if (position == -1) {
+            parent->append(this);
+        } else {
+            parent->insert(position, this);
+        }
+    }
+}
+
+int AbstractDecklistNode::depth() const
+{
+    if (parent) {
+        return parent->depth() + 1;
+    } else {
+        return 0;
+    }
+}