Prompts the user to choose a destination file and then asynchronously
saves the image under the cursor to the destination file.

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()

Returns a new EActivity for an EWebView-related asynchronous operation,
and emits a "new-activity" signal.  By default the EActivity comes loaded
with a GCancellable and sets the EWebView itself as the alert sink (which
means alerts are displayed directly in the content area).  The signal
emission allows the EActivity to be further customized and/or tracked by
the application.

Asynchronously requests data at a URI by way of a SoupRequest to WebKit's
default SoupSession, incorporating both e_web_view_redirect_uri() and the
custom request handlers installed via e_web_view_install_request_handler().

New functions:

  e_web_view_request()
  e_web_view_request_finish()

Attempts to derive a suggested filename from the given URI for use in a
"Save As" dialog.

By default the suggested filename is the last path segment of the given
URI (the unless the given URI looks like a query), but subclasses can use
other mechanisms for custom URI schemes.  For example, "cid:" URIs in an
email message may refer to a MIME part with a suggested filename in its
Content-Disposition header.

Replaces the given URI with a redirected URI as necessary, primarily for
use with custom SoupRequest handlers.  Typically this function would be
called just prior to handing a request off to a SoupSession, such as
from a WebKitWebView "resource-request-starting" signal handler.

Case in point: EMailDisplay now implements the redirect_uri() method,
and calls it from its own "resource-request-starting" signal handler.

Trivial wrapper for webkit_web_view_get_uri().

Probably made sense while we were transitioning from GtkHTML to WebKit,
but we don't need it anymore.