Add suspend support in the player for gnome

The goal of this blueprint is to support the suspend or hibernate mode on gnome desktop only.

When the PC go to suspend or hibernate mode, and when Elisa is playing a video or a audio file, The gstreamer pipeline has to release the hardware.

The player has to:

- Backup the current position
- Put the pipeline to READY

When the PC wakes up the player has :
- Put the pipeline to pause
- Restore the backuped position
- If the file is seekable, put the pipeline to PLAY, if not, to READY.

You can take a look to http://people.freedesktop.org/~hughsient/temp/dbus-interface.html