fix: Make DataGridColumn.Header and .HeaderTemplate a DirectProperty
fix: Make DataGridColumn.Header and .HeaderTemplate a DirectProperty

this enables Binding and DynamicResource usage
jkoritzinsky/Perspex,AvaloniaUI/Avalonia,grokys/Perspex,jkoritzinsky/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,jkoritzinsky/Avalonia,jkoritzinsky/Avalonia,SuperJMN/Avalonia,jkoritzinsky/Avalonia,grokys/Perspex,SuperJMN/Avalonia,jkoritzinsky/Avalonia,AvaloniaUI/Avalonia,SuperJMN/Avalonia,AvaloniaUI/Avalonia,jkoritzinsky/Avalonia,SuperJMN/Avalonia,jkoritzinsky/Avalonia,SuperJMN/Avalonia,SuperJMN/Avalonia,SuperJMN/Avalonia