tabbed workspaces for nip2

We've been using nip2 for larger and larger projects. They've started spanning multiple workspaces --- up to 13 in one case. To make these large projects easier to manage we've extended nip2 so that a single .ws file can contain many workspaces.

These multiple workspaces show up in nip2 as tabs. On startup you now see something like this:

You can create new tabs, merge tabs, drag tabs between windows, duplicate tabs, all in the obvious way. There's one other obvious feature: columns now auto-arrange, with an animation as you drag them around. This makes managing large workspaces simpler. 

Compatibility is good. If you load a tabbed workspace into an older version of nip2, you'll only get the first tab. If you load an old workspace into the new nip2, it'll appear as a workspace with a single tab.

There are test Windows and OS X binaries, and a source-code tarball for linux if you'd like to try it out. Now is the time to ask for changes if anything is broken.

Here's nip2 running a huge workspace made for the British Museum: