Preparations for the LaTeX code generation.

 * Create a dialog that shows the generated LaTeX code.
 * Change such that the cubes are stored in a shared list.
Add code to generate LaTeX code.

 * Code generates the prime implicants for all cases with four
   variables.
 * Also add unit tests for four variable cases.
 * Not tested with three or two variables, even though it *should*
   work. Unit tests should be added.
 * The code could probably be refactored a bit.