fix #939 Overlay behaviour when body is scrollable

Loading indicators associated to an element are supposed to be cover and be cenetered only in the visible portion of the element.
This worked correctly apart from the case in which the body of the document has scrollbars. In that case, the overlay was not constrained to the viewport.

This commit fixes the issue by always constraining the overley in the viewport.

Furthermore, two other issues have been tackled by this commit:

* loading indicators were not being refreshed on window resize
* The message displayed underneath the spinner could end up outside the element, or was simply mispositioned in early versions of IE.
fix #936 Cross icon is no longer displayed for input widgets
fix #933 mispositioned labels in TextArea in IE7

This commit fixes the display:inline-block bug in IE7 that is affecting the label position for the TextArea widget.
feat #921 aria:List widget refresh on datamodel change

Now when you use aria.utils.Json.add and aria.utils.Json.removeAt functions to add items to an aria:List widget (or to remove one), the list template is refreshed and it displays the new item.
doc Improve Aria.load method documentation

Close #943
Always register a binding to section processing

If section processing is not bound to any data model, a spinner overlay created using setProcessingIndicator on the SectionWrapper does not follow it as one of its parent is moved and not closed if the section is deleted. Thus if the user does not specify any binding the overlay remains at its initial place forever.

Binding it to a dummy container allows both of the shortcomings to be solved: spinner follows its parent section and is closed upon section removal.