Gst Capture: image capture fixes

Report camera is ready to capture only in image capture modes;
Emit capture failed if camera not configured for capture

Change-Id: Ia2b92e2f8c760db948185a29c3e5ceae81dd3af4
Reviewed-by: Lev Zelenskiy <lev.zelenskiy@nokia.com>
Reviewed-by: Ling Hu <ling.hu@nokia.com>

src/plugins/gstreamer/mediacapture/qgstreamercapturesession.h

@@ -127,6 +127,8 @@ public:
     void captureImage(int requestId, const QString &fileName);
 
     State state() const;
+    State pendingState() const;
+
     qint64 duration() const;
     bool isMuted() const { return m_muted; }