[Fix] Starfield spawn point fixed, it's not always in the center of the given bounding box
[Feature #6] Loading screen implemented.
[Feature #6] Escape control improved.

In the loading screen : Skip the loading
In the title menu or game over menu : Quit the game
In any other menu : Go back to game
In the game : Pause menu
[Feature #6] Start button behaviour modified

On loading screen: Skip loading
On pause or upgrade menu: Go back to game
On title screen or game over screen: Do the current button action
During game: Pause
Merge branch 'feature/loading-screen' into develop