[iOS] Option to automatically generate icons and launch screens

[Calinou]

3.x cherry-pick of #34140.

Note that generate_missing is still disabled by default, like in its master counterpart. I'm not sure whether we should enable it by default. cc @bruvzg

[akien-mga]

Thanks!

[vini-guerrero]

Hello!
I'm testing it with this PR's Github actions build...

Test Environment:
MacBook Pro 13 - 2018 Big Sur Version 11.4
Xcode 12.4 - Build version 12D4e

Is the idea of the feature to generate only-blank placeholder images or scale down from a certain input image?
If so, is there an expected min-size image to be used as an input for auto generation?

XCode Proj was generated with blank images

Then I tried to build it, but XCode got a parse error to open the project files. ( Possibly unrelated to the icons, but to my custom export templates )

Also, I've only marked Required Icons in Editor

Attached test project here.
godot-game-template.zip

[alexzheng]

I wonder why there are still required icons need to set.
Generate missing icons won't generate all these icons for iOS project?
It would be much straightforward to just set a 1024 X 1024 icon, and generate all other size.

[Calinou]

I wonder why there are still required icons need to set. Generate missing icons won't generate all these icons for iOS project? It would be much straightforward to just set a 1024 X 1024 icon, and generate all other size.

See discussion in #54010.