Main Table: Add support for section group titles

Just in case customers add more content to the main menu and want to have group names
DataNode: Initialize strings as empty

We don't want setters to be returning nil strings. Clients assume empty string.
TBXML: Fix "analyze" memory errors

Upgraded to new version of Xcode, apparently it has some tighter memory checks.
- Fix potential null dereference
- Fix potential uninitialized access