add drag and drop onto app icon (dock etc.) support for MacOS platform.

common/folder.cpp

@@ -0,0 +1,19 @@
+
+#include "folder.h"
+
+Folder::Folder(const Folder &folder)
+{
+    operator=(folder);
+}
+
+Folder &Folder::operator =(const Folder &other)
+{
+    LibraryItem::operator =(other);
+
+    this->knownParent = other.knownParent;
+    this->knownId = other.knownId;
+    this->finished = other.finished;
+    this->completed = other.completed;
+
+    return *this;
+}