Update to Bevy 0.15

[SSebigo]

Most of the tutorial is still relevant to the current version, but some would benefit from a little update. Will try to make a PR if I have time.

[SSebigo]

Well, since I'm not that familiar with Bevy, but I want to learn it, I will upgrade gradually from 0.12 -> 0.13 -> 0.14 -> 0.15. It should be better so things that weren't updated in the last release are still up-to-date with the last changes.
It's going to take some time, but it's a good challenge.

[EdEinn]

you are my hero!

[prx0]

@SSebigo I'm also interested to do a PR to upgrade some tutorials so maybe we could work together.
Especially I'm looking for updating parts who mentions deprecated bundles usages like SpriteBundle, Camera2dBundle etc.
I would like to publish a PR if I have time too.