crossword: treat blank squares as blocks for clue numbering

[davide125]

Upstreaming a fix from https://github.com/century-arcade/xd, which vendors a copy of this library. See https://github.com/century-arcade/xd/commit/dc5a6cf12c396b4c4716b73079da0174e3e07d41 and https://github.com/century-arcade/xd/issues/63 for reference.

[svisser]

Hi, it's been a while since I worked on this project but could you perhaps clarify why this change is correct? I looked at a few sources (one, two) but these refer to the use of - for empty cells, not :, which is what this pull request is implementing. Is using this character indeed part of the file format for .puz files or am I perhaps misunderstanding why this change is made?