[DMN Designer] All GRIDS: Add support for resizing columns using header

Requested by etirelli:

On the tables, it seems I can only resize columns by click/dragging borders on the body of the table, not the header.

This should probably be changed in uberfire-wires-core-grids to support column resize when hovering over the header divider (of all cells or cell boundaries? - i.e. if a header cell is merged then should we allow resizing only at the boundaries with other columns or at the boundary of where the virtual cell would be). This is likely to interfere with header cell editing and possibly column re-ordering as the mouse-down, mouse-up will be interpreted as a mouse-click.