Use activeElement instead of target.

Fixes more Firefox edge cases related to focus and selections.
Use `position: fixed` to prevent the page from jumping

Before, when you invoked `dom-paste`, the browser would scroll
to the bottom of the page, since that's where the new
"contenteditable" DIV was getting inserted.

Using `position: fixed` ensures that the DIV is on-screen,
as well as some additonal CSS to ensure that it's completely hidden.