Naming review: use more consistent names through the project

 * compositor.js has been cleaned up and is now renamed as "WindowManagerServices.js".
 * The compositor create a "window", which is the basic Wayland window exposed an an Item
 * The WindowManager instance is now named "windowManager" instead of "root"
 * The window manager wraps that window in a "windowWrapper", which contains informations about the current state of the window. The "windowContainer" and "appWindow" namings don't exist anymore.
 * The rest of the project always uses the windowWrapper as a basic input.
 * Some documentation about the respective roles of Compositor and WindowManager has been added in main.qml

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