#1086 canvas::sqmap limitations

[Andreas Kupries]

The items used to show the images in their cells are currently visible to the user of the canvas::sqmap, allowing hacks and tricks, and possibly confusing things, like when finding nearest/overlapping items.
Fix this before going v1. Basically have to inspect all methods taking and returning items and filter the internal items out. Ditto for tags, our tag for the image items should not be visible outside.