Add desktoplayouttool

This Merge Request simply adds the originally-for-redesign-MR addition that got scrapped from that merge request in favour of being its own merge request - the Desktop Layout tool.

This tool is literally just lookandfeeltool except Appearance Settings are disabled and Layout Settings are always enabled, as the name implies. Essentially, it's a means of allowing changing Desktop Layouts, without affecting the user's appearance settings, via a Terminal command.

Additionally, this MR introduces 'ToolShared' for sharing code between both tools that is consistent across the two of them.