Ephemeris visualization improvements

[Atque]

This multi-fix introduces a few new features to the ephemeris tab of the AstroCalc:

1. Letting users label ephemeris markers in any combination of year, month, days, hours, minutes and seconds by corresponding checkboxes
2. Introduce anti-clutter for both markers and labels, with configurable pixel limit
3. Letting users label only the first of each month and having the label written as three-letter-abbreviation of the month's name
4. The most complex feature: introduce a new time step "Sun at altitude", which lets users plot ephemerides of objects at a specific solar altitude, either in evenings (after noon) or mornings (before noon). It should work even for Arctic and Antarctic locations where the Sun doesn't meet conditions specified. In such cases, no ephemeris marker is drawn for that date. (Note that horizontal coordinates need to be checked for this feature to work as intended!)

Fixes #1315, #2829 and #4400

Screenshots:

The extended ephemeris extra settings window (with ephemeris in the background labeling only the first of each month):

Venus at solar altitude = -6 during this spring's evening from my location (also showing the new time step "Sun at altitude"):

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

Test Configuration:
Ubuntu 24.04.4 LTS
Qt 6.4.2.

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]

YAY! Another long-standing plan of mine, esp. the second picture.
In the GUI, you could exchange the checkbox by a Combo to switch Morning/Evening. A bit more work, but prettier IMHO.