usability and features

[StefanoCecere]

after one day workshop with kids (10/12y) these are my main requests to allow them do next GGJ (maybe i'll submit some PR if you agree):

1. move the Playground components scripts in a custom menu
2. improve the wander script with container area (or radius)
3. create a generic bullet that decreases 1 health live to the enemy, destroying it at 0 lives. this allows bosses and different enemies
4. set Gravity to 0 as default always
5. check integration with Fungus framework ( https://github.com/snozbot/fungus/ ) to not reinvent the wheel for so many features like dialogs and animations)