flickr: Use SSL

Fixes: https://bugzilla.gnome.org/730810
Prepare 3.13.2
ewsclient: Autodiscover XML can contain multiple Protocol elements

The EWS backend server does not always send the data we need in the
first <Protocol> node, so we need to keep searching if the first one
does not have the required values.

https://bugzilla.gnome.org/show_bug.cgi?id=730041
provider: Use g_type_ensure for registering types

Fixes: https://bugzilla.gnome.org/729173
Consolidate the provider types and extension point names

It makes obvious sense to use the name of the extension point as the
provider type. So lets define a single macro instead of replicating the
string literal all over the code.

This will also be useful when we implement the whitelist because the
same strings can be used in the GSettings key.

Fixes: https://bugzilla.gnome.org/729173
ewsclient: Correctly cancel pending messages on success

We can not use SOUP_STATUS_NONE with soup_session_cancel_message.
Instead we can use SOUP_STATUS_CANCELLED, as before, and keep track of
the number of pending messages so that we can finish the asynchronous
operation after all the messages have been accounted for.

Fallout from 27e86279bbaff430fe0757f97c97c44d190b7da6

Fixes: https://bugzilla.gnome.org/699553