Sub-task for SoK 2018

Understand the present implementation of checking correct answer and fix issues related to not accepting drag and drop in some regions, and improve the accuracy of drag and drop.

--> Wagons in the upper part of wagons grid accepts both click and drag which should not be the case.
--> Dropping of selected wagons is not accepted in the uppermost part of wagon grid.
--> Check that every part of wagons grid accepts drag and drop.
--> Check that top row(question row) accepts drag & drop and handles deletion of selected wagons correctly.