[Grid] Clipboard does not work with grouped columns

[mbechev]

Describe the bug
All columns that are grouped (wrapped in the <kendo-grid-column-group> tag) are excluded from the clipboard data. On paste there is no indication which columns are affected as well.

The clipboardData includes only the cell values of columns that are not grouped (the grouped columns are ignored).

To Reproduce
https://stackblitz.com/edit/angular-twsyxj