Example running a dialog before the main window

[chapulina]

🎉 New feature

• Part of Quick start dialog gz-sim#1252

Summary

Added a standalone example for how to run a custom dialog before opening the main window.

TODO

I created the new Application::CreateMainWindow function as a public version of InitializeMainWindow, and I must be doing something silly wrong because the example fails to compile with "undefined reference". That needs to be sorted out before merging this PR.

The issue above fixed itself 🤷🏽‍♀️

Test it

Follow the instructions in the example's README

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• 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 #407 (ca622af) into ign-gui3 (9b27bec) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           ign-gui3     #407      +/-   ##
============================================
+ Coverage     66.10%   66.12%   +0.02%     
============================================
  Files            29       29              
  Lines          3216     3218       +2     
============================================
+ Hits           2126     2128       +2     
  Misses         1090     1090              

Impacted Files	Coverage Δ
src/Application.cc	87.50% <100.00%> (+0.06%)	⬆️

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[mayman99]

The InitializeMainWindow seems to still be public here, but I guess this is okay because it is changed in #418