Add menu item with export functionality

Add a button to the menu for exporting a sudoku in the format that can be read by https://sudoku-solutions.com/

e.g.

123 46  8
2   4
3
4      7
5  3
      5
7 2 
8
9     4

It looks like this:

• I'm using my own branch in my local copy. Ej, I want to merge myuser/opensudoku:my-new-contribution into develop.
• All commits are signed.
• I agree to release my code and all other changes of this MR under the GNU GPLv3 license.