Make `RowCellWalker` fluent

[jferard]

...and add firstmethod.

All methods should return this (except has....

[jferard]

Same goes for RowCellWalker.

[jferard]

The main issue with this idea is that the interface RowCellWalker won't inherit from TableCell anymore: TableCell methods do not return anything, while the corresponding RowCellWalker methods would return this. I make this an unplanned issue.