Fixes #2797 - Refactor Color/TrueColor - Make TrueColor default

[tig]

Fixes #2797 - Refactor Color/True Color - Make TrueColor default
Partial fix for #457 -

Plan

• Color becomes a class that represents a 32-bit color value accessible via R, G, and B properties (there will be an un-used A property as well given that most color systems include an alpha channel, even though there's no standard for that in most terminals).

  • Color will have properties for each of the 16 legacy color names (in the current Color enum). This enables most existing app code that uses Color.White etc... to work.
  • Color will provide a way to convert between legacy-16-color colors and 24-bit colors (Value will return (this.A<< 24) | (this.R<< 16) | (this.G<< 8) | tc.B).

• Attribute will revert to looking more like it did in v1 with just a Foreground and Background property (the TrueColorForeground/Background properties will be removed.

  • Clean up extra methods that are no longer needed/redundant
    • Simplify constructors and remove duplicate code
    • Remove Attribute.Make - use new instead.
    • Remove Attribute.Get - Was only used in one unit test
    • Remove Driver.GetColors - not needed anymore
    • Remove HasValidColors stuff - With TrueColor and the map of color names to RGB values now in Color, it's not needed.
    • Remove Initalized stuff - With TrueColor and the map of color names to RGB values now in Color, it's not needed.

• TrueColor should be the default; fallback to 16 color mode would be automatic if possible.

  • If not, users should be able to force it via ConfigurationManager. E.g. "Application.Force16Colors : true.

• Persistence of colors to ConfigurationManager needs to be sorted out.

  • Json schema: Supports reading of #RGB, #RRGGBB, #RGBA, #RRGGBBAA, rgb(r,g,b), rgb(r,g,b,a), or a ColorName.
    • Alpha read but never used
    • Alpha is never written - this can be revisited someday
  • Still need to figure out how the ColorNames to RGB values map can be configurable.
    • New configurable property: Color.Colors 👍

  "Colors": {
    "White": "#F2F2F2",
    "BrightYellow": "#F9F1A5",
    "BrightCyan": "#61D6D6",
    "Cyan": "#3A96DD",
    "BrightMagenta": "#B4009E",
    "Magenta": "#881798",
    "BrightGreen": "#16C60C",
    "DarkGray": "#767676",
    "Green": "#13A10E",
    "BrightBlue": "#3B78FF",
    "Yellow": "#804020",
    "Blue": "#0037DA",
    "Black": "#0C0C0C",
    "BrightRed": "#E74856",
    "Gray": "#CCCCCC",
    "Red": "#C50F1F"
  },

- [ ] Somehow read these from the environment at startup?

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
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• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[tig]

@BDisp and @tznind - please review!

I've got a few things to clean up, but this is almost ready.

[tig]

I think this is ready!

[tig]

Working on ColorPicker... <3

[tznind]

Awesome work! I have left a few comments, nothing major.