StelPainter: More label placement options

[gzotti]

Is your feature request related to a problem? Please describe.

As found in a long discussion (#4493), StelPainter::drawText() has issues with accurate label placement.

Describe the solution you'd like Some fixes around screen/font scaling are still needed, and in principle, options for attaching either corner or center of the labels to the target point would be desirable.

While we are at it: The GravityLabels could be amended as well: Currently strings are cut into 1-letter QStrings and placed in a circle. This totally breaks for Arab, where letter forms depend on string context. Given that all text is written into a texture and then painted into the screen, such a texture created with the string with all context could be stretched as a whole.

Describe alternatives you've considered

The pragmatic solution to #4493 was manual upscaling of the screen shift. But this is ugly and is easily forgotten, leading to errors and long discussions.

Additional context See #4493

[github-actions[bot]]

Hello @gzotti!

Thank you for suggesting this enhancement.