Grid: Refactor cell creation methods to be directly callable/extensible
Grid: Refactor cell creation methods to be directly callable/extensible

This refactors logic from the inner function in `renderRow` into
`_createBodyRowCell`, and from the inner funtion in `renderHeader` into
`_createHeaderRowCell`.

maier49/dgrid,maier49/dgrid,kfranqueiro/dgrid,maier49/dgrid,dylans/dgrid,dylans/dgrid,kfranqueiro/dgrid,kfranqueiro/dgrid,dylans/dgrid