Fixed: appkit_tag_dom_elements support by CPImageView.

Without this fix, the `appkit_tag_dom_elements` flag would only affect CPImageViews unarchived from a coder (e.g. in a collection view, from a cib). It would have no effect on a CPImageView created in code.

This change refactors the _DOMImageElement code to a shared method, reducing redundancy while fixing the bug.
Simplify CPImageView code.
New: appkit_tag_dom_elements now tags with UIDs too.

Tagging with UIDs can make it much easier to debug encoding/decoding related bugs where it's not clear that the right view ended up in the right spot.

The new data attribute is called "data-cappuccino-uid".

Also collect and simplify appkit_tag_dom_elements code a little.