Add "hide all but one" occlusion mode.

[jariji]

This PR adds the "hide all but one" occlusion mode. An example use case is a note containing a collection of pairs of selections, where each selection is the prompt for the other in its pair.

For example, given a table like

small	big
a	A
b	B
c	C

in each card, five letters are occluded, and one is shown. The user is prompted to state the occluded symbol that is adjacent to the shown symbol.

[abdnh]

Code looks good. I find the behavior confusing honestly, and I vote for including tooltips explaining the different options if this gets accepted.