Fix legend position for older projects

Description

Description: With !479 (merged) the Custom(3) Element was reused as Relative. Now when loading older projects relative is used which is not correct anymore. Custom is the same as now center.

Conformity

• [ ] Changelog entry
• [ ] Unit tests
• [ ] Update INSTALL
• [ ] Downport
• Fixes: #878 (closed)

When external dependencies are removed

• [ ] Reporting to ...