Add padding qr-codes to last page

[dmitry-monakhov]

Usually last page is half empty, It would be nice to fill it up with extra copies.
I was not able to create PR, so please review proposed changes in my branch master...dmonakhov:qr-backup:add-last-page-padding

[za3k]

You should be able to make a PR. Other people have.

The code seems simple and I understand the idea. But it does add a little complexity, too. Let me think about whether this is a good idea.

If parity codes are turned on, the padding QRs should probably be new parity codes rather than copies of existing codes.