add minimal word length setting

[rspilker]

Adds a new setting to the setup screen to select the minimal word length.

Shorter words entered by the user will not be checked. The grid above the board displays a white 'x' instead of the number and the results page will not display shorter words.

Current possible values as '2+', '3+' and '4+'.

[bennorichters]

I like the idea. But I have some issues with this:

• It breaks the unit tests. They do not compile anymore. They would need to be changed to cover the new behavior anyway.
• With longer minimal length of allowed words, chances increase that there are no solutions at all. Does this need to be addressed?
• I think you found the best possible icon for the slider, but it is definitely less intuitive than the other two. But this is not a deal breaker for me.

[rspilker]

I'll fix the tests. Forgot about those. Possibly, we need to regenerate the board of there are no or not enough solutions

[rspilker]

The tests are fixed now

[rspilker]

The commits in between are already in master. The only relevant changeset is 4c9e056