[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.
It might be better to add the image part locations to 'htmlExtraPartsLoc' as well as to 'htmlImageLoc', so this can just return the former, and they will be in the expected order.  Also, I think a better name would be 'inlineContentLoc' (and 'findInlineContentLocations').

However, if you think there's no benefit to preserving the original order, I'm ok with this version too.