Fix GravityLabels with mixed-directional labels (and higher-level Unicode)

[gzotti]

Description

The multipart cultural label causes problems with gravityLabels when

• it contains characters not covered in 16-bit strings (e.g. Cuneiform): characters missing.
• it mixes Latin and right-to-left script like Arab: Arab characters are shown reversed

TODO:

• split string into the isolated Unicode strings, handle each part separately.
• Convert to u32string and process from there
• make sure multi-part labels are RTL-fit

Fixes # (issue)

Screenshots (if appropriate):

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• This change requires a documentation update
• Housekeeping

How Has This Been Tested?

Test Configuration:

• Operating system: Windows 11
• Graphics Card: irrelevant

Checklist:

• My code follows the code style of this project.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (header file)
• I have updated the respective chapter in the Stellarium User Guide
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[github-actions]

Great PR! Please pay attention to the following items before merging:

Files matching src/**/*.cpp:

• Are possibly unused includes removed?

This is an automatically generated QA checklist based on modified files.

[gzotti]

Apart from code cleanup, I think functionality is working.
@alex-w not sure, put this still into 25.3?
@kajaji is RTL for multipart labels correct?

[alex-w]

@alex-w not sure, put this still into 25.3?

Currently 25.4...

[gzotti]

I know. I did not expect to have a solution by yesterday. A few comments can still be cleaned up, but else it seems to work OK. It also does not provide new translatable strings.

[github-actions]

Hello @gzotti!

Please check the fresh version (development snapshot) of Stellarium:
https://github.com/Stellarium/stellarium-data/releases/tag/weekly-snapshot

[github-actions]

Hello @gzotti!

Please check the latest stable version of Stellarium:
https://github.com/Stellarium/stellarium/releases/latest