Enable table borders by default

[10110111]

This is one more preparation step for the stellarium-skycultures format support, where the tables are normally typeset in Markdown, which then creates plain <table> tags. These tables need the borders, so this patch enables them by default.

If a table is used for laying out content, where the borders would be unwelcome, it should now use class="layout" attribute, while "nomenclature" class is no longer used. This rule is implemented in all the current sky cultures.

[github-actions]

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

Files matching skycultures/**:

• Did you remember to update po/stellarium-skycultures/POFILES.in file respectively to changes in sky cultures?
• Did you remember to update skycultures/CMakeLists.txt and skycultures/YOUR-SKYCULTURE-FOLDER/CMakeLists.txt files respectively to changes in sky cultures?
• Did you remember to update util/skycultures/po4a.config file respectively to changes in sky cultures?
• Did you remember to define classification parameter in sky cultures (see info.ini file)?
• Did you remember to define license parameter in sky cultures (see info.ini file)?
• Did you remember to define region parameter in sky culture (see info.ini file)?

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

[github-actions]

Hello @10110111!

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

[github-actions]

Hello @10110111!

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