Fix image pasting regression

Fix image pasting regression

I introduced a regression in c14b9e67 for pasting images:
If an image is on the clipboard both as PNG and HTML with just an url,
but no plain text, pasting would fail. The reason for this was that
text contents was detected (the HTML code), nd preferred, but actually
pasting it resulted in an empty string, since the HTML import could not
handle the url This error was not checked.
The solution is first to try text paste if both text and image content
is present, and then  try image paste if the text failed.