Add constants for default icons

[skttl]

Following this tweet https://twitter.com/jjowensdev/status/1439863102439411712, could we add those to core? Would make it a lot easier to reference icons in code (when setting up sections, trees etc.)

[abjerner]

While not a complete list, there is already this: https://github.com/umbraco/Umbraco-CMS/blob/v8/contrib/src/Umbraco.Core/Constants-Icons.cs

[nul800sebastiaan]

First reaction: I don't think we should, it will never be a complete list since people can install custom icons. If there's a nice way to build a list of available icons (think: modelsbuilder) then.. maybe.

[nathanwoulfe]

Maybe a starting point?

Umbraco-CMS/src/Umbraco.Web/Services/IconService.cs

Lines 29 to 32 in 405ed44

	public IList<IconModel> GetAllIcons() =>
	GetIconDictionary()
	.Select(x => new IconModel { Name = x.Key, SvgString = x.Value })
	.ToList();

This gets all known icons (including third-party/user-added), where the key is the icon filename (less the extension). How that would be converted into anything useful, I don't know, but we do have an object containing a reference to every icon. Maybe a source generator?