Fix various issues with clipboard content handling

 * Prefer image content over the text/* one.
 * In HTML mode before trying to save the image from the clipboard (to be
   later inserted into the editor) save the clipboard's text/* content as
   if we fail to save the image (not necessary by our fault - looks like
   the current web engines (WebKitGTK+, Gecko) have some problems
   copying the IMG element with SRCSET attribute - see [0]). If this
   fails then the source application can cancel the content and we could
   not fallback to at least some content.
 * Mark editor as changed when an image is inserted.

Reported as https://bugzilla.redhat.com/show_bug.cgi?id=1307205

[0] - https://bugzilla.redhat.com/show_bug.cgi?id=1307205#c10