Increase limit of aux. numbers from 4

[x-sheep]

I often receive feedback where people prefer to use platforms other than puzz.link for number puzzles, because of how pencil marks are implemented. Currently, users can place one arbitrary number in each of the 4 corners of a cell.

Benefits:

• All numbers are supported, and we are not limited to numbers between 0 and 9.
• You can logically group marks between cells, e.g. if one mark in the top-right corner seems to be correct, you can apply the top-right mark from other cells as well.

Drawbacks:

• Manually switching between every corner is tedious.
• Having to press and unpress Shift once to switch between corners is unexpected, since Shift is usually combined with another key.
• You cannot enter more than four different numbers.

We should consider adding an alternate input mode for pencil marks that allows all numbers between 0 and 9 to be marked at the same time. This should be done by just switching to pencil mode, and e.g. pressing '1' '2' '3' to enter three pencil marks. For power users we can allow 'Shift+1' to immediately enter a mark without switching to pencil mode first.

Since this does not have any of the benefits of the current system, it should be possible to turn this behavior on and off. We could possibly keep this off by default, while always allowing 'Shift+1'.