AttachmentList needs a proper size when previewing images

In one of the states we had this comment:

"limited by SlideoutPanel to maximum width anyway"

that right now is not entirely true (SlideoutPanel does not limit the size).

The effect of this was that the requested width of '1000' was
being used and made the panel bigger than needed.

There is still the issue of not expanding the panel properly that
will be fixed in another commit.

Please note that flicking the image does not work horizontally
due to SlideoutPanel's nature of also being flickable in this
direction. The only way of solving this would be to do something
along the lines of calendar/time widgets where we have a dialog
and disable the drag capability of the panel.