Add declarations for "Dock Spawn" library.

The library offers a Visual Studio-like drag/drop dock manager for
JavaScript.

http://dockspawn.com
https://github.com/coderespawn/dock-spawn

These declarations do not include some seemingly-private/internal
functions.

Authored by Drew Noakes <https://drewnoakes.com>
Add tests for "dock-spawn" library.
Updated README with new library.
Merge pull request #1781 from drewnoakes/master

Add declarations for "Dock Spawn" library.