Splashscreen customization

[TheGondos]

This PR should probably wait for after the 2024 release.
This adds support for custom load screens via a new scenario parameter "SplashScreen" in the ENVIRONMENT section :

BEGIN_ENVIRONMENT
   System Sol
   Date MJD 51982.0975169074
   SplashScreen lime Images/myawesomelogo.jpg
END_ENVIRONMENT

The first parameter is a color in 0xbbggrr format described as a CSS color (either in #hex or as a color name), followed by the image filename to be shown on the splashscreen.
The color name is case insensitive, I tested with filenames containing spaces and it works OK.
The image is scaled with the same logic as the embedded Orbiter logo.

Since I was messing with State.h, I took the opportunity to replace the error prone char arrays with std::string

End result :

[maxq519]

Why should this wait for after the 2024 release? Seems like a relatively minor feature in terms of code, but would be really cool to have.

[TheGondos]

We need to freeze features or we'll never release a new version...
I'm not really satisfied with the syntax for the color but I don't want to invest more time in this before I get some feedback on the feature as a whole.
There are some topics to address : what happens with different screen ratios, do we need to be able to move the text...

[TheGondos]

With the 2024 version now out, it's time to get the ball rolling again,
I didn't find any document describing the .scn file format to add this to. Any idea where I could put it?

[Xyon]

With the 2024 version now out, it's time to get the ball rolling again, I didn't find any document describing the .scn file format to add this to. Any idea where I could put it?

https://github.com/orbitersim/orbiter/blob/main/Doc/Orbiter%20Developer%20Manual/SCENARIO.tex ?

[TheGondos]

Rebased to latest main + documentation updated