Support iPuz v2

[crosswordnexus]

I don't think there'd be much to do here besides incrementing an integer in the code. As far as I can tell there are no meaningful differences between v1 and v2 (i.e. every v2 iPuz file I've tried to load into Xword works just fine when I edit the iPuz to make it v1)

[jpd236]

I'm not sure there's anything else concrete to track here at least in terms of having parity with the existing v1 support (aside from #174 which has a fix out for review).

There are certainly some elements of the format that aren't supported yet, though I'm not sure how many are in active use. A couple of things that might be interesting as future work:

• Support the acrostic format (essentially by converting it to the same format as a JPZ acrostic)
• Support puzzles with detached clues (like https://squarepursuit.com/2021/12/29/sqp131-alphabetical-jigsaw-1/). This would be significantly more involved, I think - the way squares.io does it is to let you type answers in the clue list to track them. I could imagine having a mode where clue list rows have text boxes and check boxes (to mark when an entry has been used) and focusing on a clue (if it's possible at all) doesn't impact grid navigation. But this seems compelling since it's actually in use, and I could see it being useful for other situations in the future, such as solving Rows Garden puzzles with Bloom clues given in alphabetical order.

[boisvert42]

Support puzzles with detached clues (like https://squarepursuit.com/2021/12/29/sqp131-alphabetical-jigsaw-1/).

Interestingly, I don't believe this is an official iPuz thing, just a squares implementation of something that people have found useful. However, if you end up implementing it, I will for Crossword Nexus as well.