Control id attribute when pasting html to editor.

[kkrzton]

When pasting html to editor, it may contain some elements with id attributes. Pasting such content may result in invalid html with more than one element with identical id values.

While pasting, pasted and existing (in editor) content should be checked for identical ids. Such conflicts should be solved. There are few possibilities:

1. Remove id attributes from pasted content.
2. Remove id attributes from content in editor.
3. Rename id attributes in pasted content (may be tricky in some cases).
4. Rename id attributes in editor content (may be tricky in some cases).
5. Do nothing, so the user should be aware that he can generate invalid html.

I think solutions 1 and 3 seems most reasonable.