* APNG: Initial refactorings of animation exporter internals
* APNG: Make ExportDialog actually able to handle multiple file formats
* APNG: Bugfix to FPS hint and such
* APNG: Refactoring: Fix file format propagation
* APNG: Make an "APNG exporter" which creates an empty PNG container
This was the testbed of the previous integration commits.
* APNG: The actual exporter!
* APNG: Remove random src/Main.tscn changes
* APNG: Format/lint
* APNG: Format & Lint, part II
