Fix some layout issues for BigPreview and Dialog

A couple of layout issues have surfaced when attempting to use the QML Dialog on Plasma Mobile, and this patch attempts to alleviate those. The BigPreview delegate no longer squeezes outside of its allowed area, and we further no longer attempt to force the Dialog itself to be larger than the available screen area.

Further, there were a couple of warnings which got introduced through the kns:// url support patch. They have no real side effects due to where they occur, but they do look scary, and warnings are not great anyway, so... this patch checks before using them, and removes the warning. This latter bit should be a separate mr, but i messed up with git lab mr and it ended up here.