[qmf] Add QMailMessagePartContainer::findInlinePartLocations()

QMailMessagePartContainer::findInlinePartLocations() returns the locations of the inline parts
in a HTML body container, only parts with a content id reference will be returned.
At this point there's no advantage in preserving the order of these since they should be referenced inside the html body(we add only sibling parts of html body). Additional function should be added to catch any other inline parts that are 'outside' of the body(inline text or image could be easily insert by a email client)