Allow flexible layouts to be re-usable

Here's a patch for review. This will go in soon but I'm hoping there are some eyeballs to catch obvious errors.

Not yet done:

1) I just realized that I haven't added the checkbox "make this reusable" like custom content for CTools has. I'll add this next patch.
2) There's a couple of additional flexible features/cleanup I need to do. That may be separate. Particularly, finer control on classes, plus checking to see if we really *need* to render the left/middle/right divs inside a column and not rendering them if so. (we may also be able to simply avoid rendering column/row divs if columns and rows are, in fact, alone. Plus I want an option to fix the canvas width, and another option to disable the 99% scaling. These probably should be a separate patch but are part of this effort.

Oh one other comment: Preview is lame right now because there's no content to preview it with. Need to make sure that provides custom content.

Updated patch allows flexible layouts to be made reusable in the layout designer.

This one turned out not quite as nice as I would've liked, but it works; the workflow is different and I am sure that will confuse people, but the option to embed a form within a form is not there, and flexible has no visiblity into the actual panels edit form.

Oh yeah. Still needs help to make the preview work.

Ok the preview button works in this patch

Those seem to all be issues with the IPE, which is pretty new in -dev and definitely not ready for prime time. There is probably no option to add node parts because there's no node context available at that point. I'll have to take a look at that when I move on to the IPE later.

I like your wording for the checkbox more, definitely, and will switch to that.

Ok, I fixed fixed the preview, cleaned up some code and committed this!