Ajax pages

So there you go, my complete refactor for AjaxPage as per #1571.

What I basically did is create a new class TextPage, that extends Page to add a string body.

I also created JSONPage and XMLPage. Both extends TextPage, but they use XMLElement and array in order to hold the data.

I deprecated AjaxPage, but left it there for backward compatibility. Also, it just extends XMLPage now.

I also adapted all content.ajax* pages to use the new classes.

Finally, I had to change a tiny bit the backend.views.js in order to reflect a small change in how handles are return in json (it was only a string, now it's a json hash/object).

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Comments on this big PR are more than welcome. I did small cohesive commits, so it should be easy to follow. I tested a lot, but I could not test it all. Please be kind and report to me anything's that's broken.