[Feature #81] Tutorials updated.

Scores have been removed and they have been thought again to be more
understandable.
[Feature #82] Loading scene added

Scene fades in, presenting the logo on a light gray screen, then fades out
to let the Welcome scene fades in.
[Feature #82] Resources are preloaded in the Loading screen
[Feature #82] One resource is loaded at a time, on each frame.
[Feature #82] Preloading enhanced.
[Improvement] Pressing back on the Loading screen quits the game
[Improvement] Loading logo repositionned higher.
Merge branch 'master' of https://github.com/aurelien-defossez/trashKawaii into game_over_in_game_scene_66

Conflicts:
	game/src/scenes/Game.lua