Clipboard > Paste: add additional failsafes for Chrome

Chrome vomits a whole bunch of information onto the clipboard, even when
"Copy Image" is specifically used.  (Which begs the question - why
include a "Copy Image" option at all, if you're just going to copy the
URL and other metadata anyway?)

To better improve PD's protection against Chrome clipboard data, I've
added an additional fallback if both PNG and URL downloading fail; this
allows images from HTTPS URLs to still be copied successfully from
Chrome.