Improve the launch of apps

 * Implement a stub "launch" LS2 service
 * start the DummyWindow through this new "launch" LS2 call.
 * create a LaunchableAppIcon class to represent an icon (with an optional title text) that can start the corresponding app when clicked.

Also, reorganize a bit the way the LS2 LunaNext service is instantiated, to have only one instance of the service.

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>