soup-session, soup-connection: update handling of "ssl" flag

Store all SoupSessionHosts with a URI using either "http" or "https"
as the scheme. Then we don't need to explicitly tell SoupConnection
whether the connection is SSL or not, since it can figure it out from
the URI.
soup-socket: Add SoupSocketProperties to transfer props from session to socket

There are a large number of SoupSession properties that need to be
propagated through the SoupConnection to the SoupSocket. Wrap them all
up into a single struct for ease of carrying.
soup-connection: don't disconnect the connection on dispose

In the normal case, SoupSession will do it itself, and in other cases
it might not be wanted.

https://bugzilla.gnome.org/show_bug.cgi?id=721343
soup-message-queue: minor fix

SoupMessageQueueItem's priority field is not private to
soup-message-queue.c.
soup-session: add soup_session_proxy_connect* functions

Add functions to connect to a remote host, possibly via an HTTP proxy.

Based on a patch from Dirkjan Ochtman.

https://bugzilla.gnome.org/show_bug.cgi?id=721343