Data model refactoring

    This refactors the Okapi TextContainer logic behind a Variant
    interface, as a scaffold for future XLIFF2 work (or tag UI work).

    Much of the logic in SegmentTextCell.SegmentFilter has been pushed
    down into the variant, since it relies on internal tag
    representations.
OC-3 - Code rendering in the data pane

    Based on the data model refactoring in the previous commit.