VideoPlayer: Stop all video playback activity when deactivating the page

If the playback is not stopped and source is not disconnected we
sometimes trigger a crsh in GStreamerVideoOutput. The stack trace for it
is:
*0  ?? () from ./usr/lib/libQt5Core.so.5
*1  QMutex::lock() () from ./usr/lib/libQt5Core.so.5
*2  QCoreApplication::postEvent(QObject*, QEvent*, int) () from ./usr/lib/libQt5Core.so.5
*3  GStreamerVideoOutput::frame_ready(_GstElement*, int, void*) () from ./usr/lib/qt5/qml/Sailfish/Media/libsailfishmediaplugin.so
*4  g_cclosure_marshal_VOID(int0_t) () from ./usr/lib/libgobject-2.0.so.0
*5  g_closure_invoke () from ./usr/lib/libgobject-2.0.so.0
*6  ?? () from ./usr/lib/libgobject-2.0.so.0
*7  g_signal_emit_valist () from ./usr/lib/libgobject-2.0.so.0
*8  g_signal_emit () from ./usr/lib/libgobject-2.0.so.0
*9  ?? () from ./usr/lib/gstreamer-0.10/libgstdroideglsink.so
*10 ?? () from ./usr/lib/libgstvideo-0.10.so.0
*11 ?? () from ./usr/lib/libgstbase-0.10.so.0
*12 ?? () from ./usr/lib/libgstbase-0.10.so.0

With this patch I can't reproduce this crash any longer.

Signed-off-by: Peter Tworek <tworaz666@gmail.com>
VideoOverview: Load page content after it has been made active

This makes the transition animation much smoother.

Signed-off-by: Peter Tworek <tworaz666@gmail.com>