Add Save and Load with built-in actions

[ClementPasteau]

No description provided.

[github-actions]

Preview the game(s) changed or added in this Pull Request

• https://editor.gdevelop.io?project=https://raw.githubusercontent.com/GDevelopApp/GDevelop-examples/save-load/examples/save-load-simple/save-load-simple.json

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This is an automatic message displaying links to the games in this PR - double check the JSON in case of doubt.

[github-actions]

Images automagically compressed by Calibre's image-actions ✨

Compression reduced images by 49.4%, saving 3.8 KB.

Filename	Before	After	Improvement	Visual comparison
examples/save-load-simple/assets/StartingPlayer.png	1.6 KB	547 B	67.2%	View diff
examples/save-load-simple/assets/StartingGround.png	1.4 KB	414 B	70.4%	View diff
examples/save-load-simple/assets/Flat dark joystick border.png	1.6 KB	939 B	42.5%	View diff
examples/save-load-simple/assets/Flat dark joystick thumb.png	1.0 KB	669 B	35.9%	View diff
examples/save-load-simple/assets/StartingCoin.png	654 B	350 B	46.5%	View diff
examples/save-load-simple/assets/Top arrow button.png	926 B	641 B	30.8%	View diff
examples/save-load-simple/assets/Background.png	277 B	198 B	28.5%	View diff
examples/save-load-simple/assets/Large sign.png	232 B	207 B	10.8%	View diff

[github-actions]

Images automagically compressed by Calibre's image-actions ✨

Compression reduced images by 7.2%, saving 73 B.

Filename	Before	After	Improvement	Visual comparison
examples/save-load-simple/assets/Flat dark joystick thumb.png	669 B	614 B	8.2%	View diff
examples/save-load-simple/assets/StartingCoin.png	350 B	332 B	5.1%	View diff

6 images did not require optimisation.