Editor: hotfix default ListBox.SelectedBackgroundColor

After we changed to storing full RGB as color indexes, controls no longer store palette indexes in their properties in 32-bit games, but encoded R8G8B8.
Unfortunately, there's still some old logic in AGS engine, where it will consider color value 0 to be transparent.

Either we need to revise that logic, or store color properties as A8R8G8B8 with opaque alpha.
In regards to the second approach, first will need to investigate what more changes that will require, and if there are any potential problems.
But even if we do that, there's still a need to distinguish black and transparent color for GUI elements.

Editor/AGS.Types/GUIListBox.cs

@@ -22,7 +22,10 @@ public GUIListBox(int x, int y, int width, int height)
             _showScrollArrows = true;
             _textColor = 0;
             _selectedTextColor = 7;
-            _selectedBackgroundColor = 16;
+            // FIXME: selected bg color was 16, but had to change to 17 as a temp hotfix,
+            // because 16 is pure black (ags color property = 0), and ListBox refuses to
+            // paint selection if property eq 0.
+            _selectedBackgroundColor = 17;
             _textAlignment = HorizontalAlignment.Left;
         }