Enhancement
                         
                    
                    TL offers the option of defining additional headers for table columns. This is currently not yet possible inApp. The following use cases must be considered:
- It should be possible to add additional headers for known columns.
 - The additional headers should be able to be HTML fragments.
 - Existing additional headers should be retained.
 
Solutions/ideas for this: New column configuration for additional headers:
- New column configuration "Additional headers" in which a list of HTMLFragmentProviders can be configured
 - The configuration should be implemented as a ColumnConfigurator
 - The existing headers should be retained in the configuration
 
Test
s.h. demoTreeGrid.layout.xml