Asynchronously copies the image under the cursor to the clipboard.
This replaces the "cursor-image" property, which attempts to match the
image URI to a subresource from WebKitWebDataSource. The problem with
that approach is EMailDisplay redirects several URI schemes to its own
custom request handlers which further mutate the URI. So for example,
a text/html message may use a "cid:" URI to refer to an embedded image,
which EMailDisplay transforms to a "mail:" URI and finally to a "data:"
URI. The final image URI might not be derivable from the original URI
without retracing the SoupRequest.
The "image-copy" action now calls e_web_view_cursor_image_copy().
This also adds an explicit requirement on gdk-pixbuf-2.0 >= 2.24 for
gdk_pixbuf_new_from_stream_async() / finish().
New functions:
e_web_view_cursor_image_copy()
Removed functions:
e_web_view_get_cursor_image()
e_web_view_set_cursor_image()
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