Add support for pasting plain text with line breaks

This patch is now ready to be merged once it passes review.

The current implementation always pastes new content as a single long string. This feels significantly unfinished. Adding support for paste with newlines greatly improves the paste experience, so it at least feels like a simple editor.

Note, this does not add support for pasting with styles, or pasting non-plain-text elements such as images or table content. Pasting into lists is supported by use of the split paragraph operation.