Support Database Custom Data Merging (#3002)

* Introduce _LAST_MODIFIED custom data entry that stores the last modified datetime of the database's custom data entries
* Merge custom data from source database to target
* Modify tests to be aware of _LAST_MODIFIED entry

tests/TestMerge.h

@@ -59,6 +59,7 @@ private slots:
     void testMergeCustomIcons();
     void testMergeDuplicateCustomIcons();
     void testMetadata();
+    void testCustomdata();
     void testDeletedEntry();
     void testDeletedGroup();
     void testDeletedRevertedEntry();