Anki-style "AddPairsDialog"; training keys

trainingdialog - keyboard buttons “1” and “2” now answer Moo and Quack
in an idiot-proof and error-free way.

filewindow - AddPairsDialog allows user to add new minimal pairs to the
corresponding table.
Cosmetics not yet applied (not beautiful yet, funnily positioned
widgets) but the functionality is there)