Test and improve pasting

I just did a global search for similar arrays and it appears four times... although on occasion it's slightly different, for example, `"LI"` isn't usually included and in some cases `"DIV"` is included... Is this something that should be normalized and used everywhere? `constants.js` defines `BLOCK_ELEMENTS` as ` ["H1", "H2", "H3", "H4", "H5", "H6", "P", "PRE", "DIV", "BLOCKQUOTE"]` and as far as I can see, `Constants.BLOCK_ELEMENTS` is only ever referenced *once* in the entire wysiwyg. Sorta weird...