Add a built-in screencast recording facility

For development and demonstration purposes, it's neat to be able to
record a screencast of gnome-shell without any external setup.
Built-in recording can also give much better quality than is possible
with a generic desktop recording, since we hook right into the paint
loop.

src/shell-recorder.[ch]: A general-purposes object to record a Clutter
 stage to a GStreamer stream.
src/shell-recorder-src.[ch]: A simple GStreamer source element (similar
 to appsrc in the most recent versions of GStreamer) for injecting
 captured data into a GStreamer pipeline.
src/test-recorder.c: Test program that records a simple animation.

configure.ac src/Makefile.am: Add machinery to conditionally build
 ShellRecorder.
tools/build/gnome-shell-build-setup.sh: Add gstreamer packages
 to the list of required packages for Fedora.

js/ui/main.js: Hook up the recorder to a MetaScreen ::toggle-recording
 keybinding.

http://bugzilla.gnome.org/show_bug.cgi?id=575290

js/ui/main.js

@@ -20,6 +20,7 @@ let overlay = null;
 let overlayActive = false;
 let runDialog = null;
 let wm = null;
+let recorder = null;
 
 function start() {
     let global = Shell.Global.get();
@@ -71,6 +72,24 @@ function start() {
             show_overlay();
         }
     };
+
+    global.screen.connect('toggle-recording', function() {
+        if (recorder == null) {
+            // We have to initialize GStreamer first. This isn't done
+            // inside ShellRecorder to make it usable inside projects
+            // with other usage of GStreamer.
+            let Gst = imports.gi.Gst;
+            Gst.init(null, null);
+            recorder = new Shell.Recorder({ stage: global.stage });
+        }
+
+        if (recorder.is_recording()) {
+            recorder.pause();
+        } else {
+            recorder.record();
+        }
+    });
+
     display.connect('overlay-key', toggleOverlay);
     global.connect('panel-main-menu', toggleOverlay);