EMailSession: Remove leftover EProxy instance.
EGravatarPhotoSource: Remove EProxy usage.

EProxy is deprecated.  No easy access to ESourceRegistry from which to
grab the "system-proxy" ESource, so just rely on SoupSession's default
GProxyResolver.  Good enough; this is a nice-to-have feature anyway.
EMailAutoconfig: Add an ESourceRegistry property.
EMailAutoconfig: Set SoupSession's GProxyResolver.

Pass the built-in proxy ESource to SoupSession as a GProxyResolver.

Replaces EProxy usage, which is deprecated.
EShell: Configure WebKit's default SoupSession.

Configure WebKit's default SoupSession to honor our proxy settings.
EHttpRequest cleanups.
EHttpRequest: Simplify proxy handling.

When creating a temporary SoupSession, synchronize its GProxyResolver
with the SoupRequest's SoupSession which is already configured to use
our built-in proxy ESource.
Remove unused itip_publish_comp().
EMeetingStore: Remove deprecated EProxy usage.

EMeetingStore needs an ESourceRegistry property so it can obtain
an appropriate proxy ESource to serve as a GProxyResolver for the
SoupSession.

But frankly, this code is a mess and I can't be bothered right now.
The SoupSession will just use the desktop-wide proxy settings which
is fine for the vast majority of users.