Add submenu for GameResource

[bub-bl]

Add a way to add few game resource in the same category

[GameResource(Category = "MyCategory", ...)] does not working

[bub-bl]

@garrynewman

I can't create a pull request with code for this fix on https://github.com/sboxgame
This code fix this issue:

• Go in file \sbox\addons\tools\code\CreateAsset.cs
• Go at line 84
• Replace the following code:

                        menu.AddOption( $"New {gameResource.Name}..", gameResource.Icon, () =>
			{
				var fd = new FileDialog( null );
				fd.Title = $"Create {gameResource.Name}";
				fd.Directory = folder.FullName;
				fd.DefaultSuffix = $".{gameResource.Extension}";
				fd.SelectFile( $"untitled.{gameResource.Extension}" );
				fd.SetFindFile();
				fd.SetModeSave();
				fd.SetNameFilter( $"{gameResource.Name} (*.{gameResource.Extension})" );
			
				if ( !fd.Execute() )
					return;
			
				CreateNew( fd.SelectedFile );
			
			} );

• With this code

                        // Create sub menu from category propery if is not empty
			var subMenu = string.IsNullOrWhiteSpace(gameResource.Category)
				? menu
				: menu.FindOrCreateMenu(gameResource.Category);
			
			subMenu.AddOption( $"New {gameResource.Name}..", gameResource.Icon, () =>
			{
				var fd = new FileDialog( null );
				fd.Title = $"Create {gameResource.Name}";
				fd.Directory = folder.FullName;
				fd.DefaultSuffix = $".{gameResource.Extension}";
				fd.SelectFile( $"untitled.{gameResource.Extension}" );
				fd.SetFindFile();
				fd.SetModeSave();
				fd.SetNameFilter( $"{gameResource.Name} (*.{gameResource.Extension})" );

				if ( !fd.Execute() )
					return;

				CreateNew( fd.SelectedFile );
			} );