UNDO Placeables: Cycle forward through existing placeables using keyboard shortcut

Mimics Virtaal behavior:

* If the last placeable is selected then all placeables are unselected,
* If no placeable is selected then selects the first one,
* If any other placeable is selected then selects the next one.
UNDO Placeables: Cycle back through existing placeables using keyboard shortcut

Mimics Virtaal behavior:

* If the first placeable is selected then all placeables are unselected,
* If no placeable is selected then selects the last one,
* If any other placeable is selected then selects the previous one.