Add "pencil in" option

[hoelzro]

From hook in the app comments:

I miss the 'pencil in' option. If that was added, it'd be awesome :)
Let's say I know that in a square there is either 2, 4, or 5, but I cannot tell yet which one. The feature I propose is that you can "pencil in" those numbers for now instead of deciding which one of them to write with "ink".
Some other sudoku apps have that already and usually they then write the "pencilled in" numbers small into the square. Then you can erase each number when you decide it's not an option anymore, or just overwrite the whole field by "inking in" your decision.
A good example how this can be done (also for many other advanced features) is KSudoku from KDE.