added support for Model migrations

now we have a table core.migration and each model (User, Group, Item, ItemContent and a new Migration) have its own
row in the table with a version number

added to config:
db_make_migration_if_needed and db_stop_if_migration_fails (need description yet)

winixd/models/itemcontent.h

@@ -230,6 +230,15 @@ public:
 	bool CanContentBeHtmlFiltered();
 
 
+	bool do_migration(int & current_table_version);
+
+
+
+protected:
+
+	bool do_migration_to_1();
+	bool do_migration_to_2();
+	bool do_migration_to_3();
 
 };