issue with directional CU movement -- easily repro-able in Pinterest

[before starting with this, ensure that Pinterest's urlData is not commented out.]

Issue is easies to repro on Pinterest:

Try to move horizontally to from a CU where the best matching CU for the movement is outside the viewport, but there are other matching CUs within the viewport. Nothing happens.

Some notes for resolution:

1. Either the best match CU in the viewport should get selected
2. or we should scroll to the be best matching CU that is outside the viewport
3. or maybe, the current behavior (of trying to scroll the page should instead be maintained, but if there is no scroll movement (in that direction) possible on the page, then either 1) or 2) should be done.
4. Also, close modCUs_mgr: isCUFullyInViewport() and isAnyPartOfCUinViewport() to be updated to factor in the horizontal dimension as well #199 first (and consider it's implications to the problem as well)