implement event listener for notifying when translations are ready

[osma]

Reasons for creating this PR

Smarter event handling for frontend components that need to be initialized only after the translation service is ready.

See #1771

Link to relevant issue(s), if any

• Closes Add callback notification facility to the JS translation service #1771

Description of the changes in this PR

• add onTranslationReady function for registering event handlers that should be called when translations are ready
• use onTranslationReady in Vue components instead of stupid polling boilerplate code

Known problems or uncertainties in this PR

n/a

Checklist

• phpUnit tests pass locally with my changes
• I have added tests that show that the new code works, or tests are not relevant for this PR (e.g. only HTML/CSS changes)
• The PR doesn't reduce accessibility of the front-end code (e.g. tab focus, scaling to different resolutions, use of .sr-only class, color contrast)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)

[osma]

Hmm, I think there's a race condition that still needs to be fixed.

[UnniKohonen]

LGTM!

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[osma]

I added a check for the case when callbacks are registered after the translation service is initialized, and made sure those are called immediately. I tested it by making a 1000ms delayed call to onTranslationReady and it seemed to work. AFAICT, this special case can't happen with the current onTranslationReady calls we have, but it might happen later, so it's better to be able to handle that already.

All done, merging.