New chat view using gtkwebkit

This changes the text buffer widget to a WebKitWebView so that we can
use web technologies to control the display.

This change comes with a new dependency: libwebkit2gtk-4.0. Should
this dependency not be available on the system, we can also build the
client using libwebkit2gtk-3.0. However, the links won't be clickable.

New features:
 - Implemented delivery reports.
 - Avatars are now displayed in the chat window.
 - Links in the chat window are now clickable.

When the client is launched with the -d option, you may right click on
the chat view to open up the dev tools.

In order to improve performance, one WebKitWebView is re-used for all
of the ChatViews, since we only display one at a time.

Tuleap: #1073
Change-Id: Ic945fa6c92f92e391f0362310ddc2f0fa16641bf
[stepan.salenikovich@savoirfairelinux.com: added change_view(); start
 loading webkit on window init; destroy webkit on dispose; prevent
 warning when dispose is called more than once on ChatView]
Signed-off-by: Stepan Salenikovich <stepan.salenikovich@savoirfairelinux.com>
diff --git a/src/config.h.in b/src/config.h.in
index 463f7a0..68130ef 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -7,6 +7,7 @@
 #define USE_LIBNOTIFY @USE_LIBNOTIFY@
 #define USE_APPINDICATOR @HAVE_APPINDICATOR@
 #define USE_LIBNM @USE_LIBNM@
+#define HAVE_WEBKIT2GTK4 @HAVE_WEBKIT2GTK4@
 
 #define RING_CLIENT_APP_ID "cx.ring.RingGnome"