Column cssClass specified in item's RowMetadata should override the default column's cssClass

Rationale: It is possible to define row metadata by implementing getRowMetadata function to change the default row's properties. One of the properties is cssClasses. It would be logical and consistent to provide the same level of customization for particular cells on this row by defining cssClass property in the metadata column definitions.

One of the use cases is the flexibility in formatting group rows.

In this implementation the column metadata's cssClass overrides the default column's cssClass rather than concatenating it. This gives more freedom in how cssClass is defined. If metadata's cssClass needs to be concatenated to the default cssClass it can be easily done.

Note: there is a similar pull request #823 but the implementation is insufficient.