[ign to gz] Deduplicate QML files

[chapulina]

🦟 Bug fix

• Follow up to ign -> gz Header Migration : gz-gui #395
• Part of C++ namespaces, directories and header guards from ignition to gz gazebo-tooling/release-tools#711

Summary

Here's a migration for the QML files which removes the duplicated ones. The trick is:

• include/gz/gui/qml/qmldir_deprecated adds support for the ignition.gui deprecated module, so downstream QML files calling import ignition.gui 1.0 still work
• The aliases added inside qmldir and qmldir_deprecated make sure that both Ign and Gz components are available even when importing either ignition.gui or gz.gui.

Test it

• Build and run the gz_components example - this shows that both IgnSpinBox and GzSpinBox work with import gz.gui 1.0
• Then change it to import ignition.gui 1.0 and check that this works too

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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[methylDragon]

LGTM on green CI, I'm not sure why CI isn't green though...

Also noting that this means the other Ign prefixed stuff won't work in downstream libs since the redirection aliases for them 💥 ?

(The Ign -> Gz aliases were in https://github.com/gazebosim/gz-gui/pull/449/files#diff-978089d7a839a8b0ec51044f445624a06765881a90adaa0e66a381d1326bcc3b , but now are gone)

[chapulina]

I'm not sure why CI isn't green though...

• That's caused by Use assimp for loading meshes gz-common#393. I haven't had time to look into it yet

the other Ign prefixed stuff won't work in downstream libs since the redirection aliases for them

Ah yeah, the ones outside of the module aren't meant to be used downstream, so that should be ok