Fix attribute update when <dialog> isn't present

[chapulina]

🦟 Bug fix

Summary

This fixes a situation when a file doesn't have a <dialog> element yet, and we need to create one to update its value.

I also increased the Dialog's test coverage to 100% and pre-deprecated a function that's no longer needed.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

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[codecov]

Codecov Report

Merging #455 (053c6d2) into ign-gui3 (33bbf41) will increase coverage by 0.48%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           ign-gui3     #455      +/-   ##
============================================
+ Coverage     67.81%   68.30%   +0.48%     
============================================
  Files            29       29              
  Lines          3281     3278       -3     
============================================
+ Hits           2225     2239      +14     
+ Misses         1056     1039      -17     

Impacted Files	Coverage Δ
src/Dialog.cc	100.00% <100.00%> (+26.98%)	⬆️

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[mayman99]

lgtm!
the only two downsides to the new impl is that we can only write bool attributes and we only write one by one, we can update it if such use cases arises then imho