Pencil marks

Added pencil marks, minor layout changes and redid the sudoku generation
algorithm to be much more efficient (full backtracking now, rather than
just throwing away the entire board if you reach a dead end).
More efficient solving

Decided to use kudoku library to solve as it's much quicker. Should be
so much quicker generating, digging holes and solving. Should also
create better sudokus as I modified the digging holes algorithm. Sudokus
should be have a unique solution.