label: Fix markup parsing interactions [cherry-pick from `main` to `gtk-3-24`]

We were not handling mnemonics vs markup right in all cases. Rewrite the _-stripping code to do it during the link parsing, instead of as a separate function. This avoids the issue of stripping _ from attribute names in markup.

Add tests.

Fixes: #3706 (closed)

dboles: cherrypick of mclasenʼs 875a92b9 only notable difference is expected output in test – unsure why?