Convergenza del layout e cambiamenti estetici

What I've done here:

 * Added haptic feedback to Pause button, using AbstractButton (provided by Ubuntu.Components)

 * Added a thin line below the glasses, to make the design richer and a little bit more skeuomorphic.

 * Reduce the size of Pause button, since I felt it somehow "intrusive".
      I'm still not sure about the position on the bottom-left margin. Perhaps making the button a little bigger again, or a horizontal-centered position would be a better solution.

 * Convergence:
   * Keeping the same size for the game Scene, I've anchored it to the parent center. By doing so, we won't mind anymore about the screen size of the target device.
   * Added an UbuntuShape as background for the Scene, and set "clip: true" in order to have a "well-defined" border where the game is running inside.