qmllint: Warn about using width/height in Layouts

Specifying width/height of an item in a layout does not make sense, as the layout will just overwrite it anyway. 

To control width/height there, Layouts attached properties (preferredWidth, minHeight, maxWidth, ...) should be used instead.

We cannot judge whether the specified width ought to be the preferred,min or maxWidth, but we can at least warn about its usage and point to some general documentation (documentation currently pending).