[Enhancement] Possibilty to set which translation is tested in training mode

[sfbg]

It is good to have the current setting of showing either the "from" or "to" entry.
But for learning purposes it should be possible to set if you want to have only the "from" or only the "to" language tested.

[Bu33zhidao]

Yes, that would be great!
Or e.g.: Vocabularies with a high priority are shown from both sides to assist learning; once the priority drops below some threshold only one side of the card will be presented.

[Top-Ranger]

I like the ideas - I will look what I can do

[Top-Ranger]

Added suggestion to #11