Add GIF support for loading image frames and import

[Xrayez]

Co-Authored-By: @Daw11, as originally implemented for Godot in godotengine/godot#31831.

Notable changes:

• AnimatedImage renamed to ImageFrames, to avoid confusion with AnimatedTexture, see Errors in renderer when assigning an empty image to TextureButton godotengine/godot#41106 for instance.
• It's not trivial (or even possible) to modify the canvas item editor for drag-n-drop feature to create a sprite from GIF, so these features are also removed (there are too many core changes required for that anyway).

Note that some changes are currently governed by Godot development requirements. This PR is created in hopes that it can be reviewed (and hopefully approved), so that these features can have a chance to be merged directly in Godot for the master branch (and perhaps backported to 3.2), else these features are already quite functional and can be tested, and eventually merged to Goost.

You can test this out by compiling this PR with the plain scons command.

Godot proposal: godotengine/godot-proposals#1433.

[Xrayez]

See godotengine/godot-proposals#1433 (comment).

This is mostly completed, image frames loading works and successfully importing as AnimatedTexture now. Note that current implementation is a self-contained module. The classes needed to cover other animated images formats demand more complete solution: implemented classes like ImageFrames, ImageFramesLoader, ImageFramesFormatLoader etc. are mostly agnostic to the animated image format, so these classes can be considered core and can be moved to core/ if someone wants to implement other animated image loaders, but for now it's better that those classes remain closer to a module so other people can compile this module without any dependencies.

[Xrayez]

Re-implemented SpriteFrames importer, so should be more up-to-notch now compared to godotengine/godot#31831.

The lack of decision making regarding godotengine/godot-proposals#1433 shouldn't block this PR, I cannot possibly know for sure how long it would take for the proposal to get reviewed/approved/rejected, there are no deadlines so this could take infinitely long to wait for, so lets just test the implementation "in production" and see.

Credits: I've added @Daw11 as a co-author (means that changes will be reflected in the actual git history) and hope that it's alright for these features to be used by other people considering the amount of 👍 the original PR accumulated over a year (!), so I'm going to merge this now rather than wait another year to pass. I'm inviting @Daw11 as a collaborator in case he'd like to maintain the existing and/or implement other animated image loaders, thanks for working on this! I hope my feedback was also useful in godotengine/godot#31831 the time you worked on that PR. Also added to AUTHORS.md, cheers! ❤️

Context: @Daw11 hasn't responded to any messages from the moment he has completed all the planned changes for godotengine/godot#31831 (comment), so the PR considered abandoned.

[sairam4123]

Implement APNG too, #193