Bugfixes for 2.0.0

The main difference for this is the change of ContextMenu to ContextMenuStrip. The header context menu now is opened also when clicking on any cell in the table.
If this is not desired table.ContextMenuStrip should be set to null

- Moved gitversion.yml
- Include pdbs in nuget package
- create symbol package
- Fixed nullref exception in HeaderContextMenu OnOpening in case the model is not set
- Avoid duplictate header columns in header context menu