Don't destroy currentView when rendering a LayoutView

** This is very much a WIP **

This addresses #1263 

Instead of resetting all of the regions whenever the LayoutView renders, it first tries to detach/reattach the contents of the region. If it cannot do this (either there were no contents before the render, or there is no place to put them after the render) then the region is reset, and any detached elements are actually removed.

This takes advantage of $.fn.detach, which is like remove but it preserves the events so we don't break the currentView when temporarily removing it from the DOM.

I don't think this is a good implementation, however I hope it serves as a proof of concept that can start a discussion about whether this approach (detaching/reattaching dom elements) has pitfalls.

I think a better implementation would involve modifying Region to support this behavior. Perhaps a `Region.detach` and `Region.attach` or something along those lines.

In order to preserve the ordering of render events, I removed the call to `ItemView.prototype.render.apply`. This results in a little bit of duplication.

Things I don't like about this:

* Not sure of the best way to answer the question "should we try to detach/reattach"
* LayoutView is messing with internal Region stuff which is bad.
* Duplicated if statements -- wasn't sure how to better organize this code. I think this will be solved by moving some functionality into the Region itself