Add a way to specify the default color schemes (Base, Dialog etc)

[tznind]

Currently with ColorScheme in TerminalGuiDesigner there is an assumption that any ColorScheme that matches one of the base ones is '(inherited)'. This means that user can only set ColorSchemes they have created (or default ones supplied e.g. greeonOnBlack).

User cannot set to the base schemes.

If user creates a custom scheme with the same colors as one of the base schemes then I think designer will get confused (need to check this)

Implementation of this is important but might be complicated. See :

string? GetNameForColorScheme(ColorScheme s)
public void AddOrUpdateScheme(string name, ColorScheme scheme)

[tig]

Related:

• Colors: ColorSchemes refactor - Should honor terminal color settings etc... Terminal.Gui#2381
• Colors: Update Color System (TrueColor, Blink, Underline, etc...) for v2 (Master Issue) Terminal.Gui#457