Changing aui-modal to work with bootstrap 3

This is fixing the problems that came up with the bootstrap update on aui's modal module.
Bootstrap changed the structure of modals, they now need the modal's contents to have 3 containers instead of 2 (.modal, .modal-dialog and .modal-content). This causes a problem with the way our modal's implemented, as some mixed in widgets won't work as they should anymore. For example, WidgetStdMod always inserts its contents on its contentBox, but we need those contents to be inside an element that is inside the contentBox instead. Other problems were relatd to the outtermost div now filling the whole screen in bootstrap, and so features like dragging and resizing need to be moved to other divs to work properly.

My approach for this was to divide our Modal widget into 2. The main one has .modal and .modal-dialog as its boundingBox and contentBox respectively. The other is responsible for the actual contents, which is .modal-content. I'm rendering this second widget inside of the first widget's contentBox and moving some of the code there, so it can apply to .modal-content instead of .modal.

This is not yet final, as I still need to fix unit tests and maybe organize this better, but I'm sending it so people can have a look at it and see if we can find better solutions before going further.

All the funcionality should be working on this version, except for an issue when the user resizes the modal to a small size. which makes the body/footer still show up outside of the modal. This seems to just be a bootstrap css change though, where modal-content doesn't have the 'overflow: hidden' css anymore, so we probably need to add it ourselves to make the resizing work properly.