re: New higher-performance rnote file format, and atomic file saving

[anesthetice]

The overall goal of this PR is to improve the handling of .rnote save files.

tl;dr

• Shorter loading times (~10x improvement*)
• Shorter saving times (~6x improvement*)
• Smaller file sizes (~12% reduction)
• Safer saving mechanism

* improvement factor measured only with respect to EngineSnapshot <-> bytes

Main changes:

• parallelized serialization and compression
• direct loading pathway when compatible
• for compression, using zstd instead of gzip
• atomic file saving (also applied for autosaves)

Here's a diagram of the simplified process to get from EngineSnapshot to a .rnote save file and vice-versa.

fixes #1128

Note that this PR is a re-write of #1177, some of the important changes being:

• (+) parallelism for serialization and compression
• (+) the introduction of a file version separate from the version of Rnote
• (-) scrapped save preferences (not worth complexity cost)
• (-) scrapped alternative serialization methods (for compatibility reasons)

[anesthetice]

@flxzt, @Doublonmousse, I'd say this PR is ready for review. Take all the time you need of course, and feel free to ask me for information, changes, etc. 🫡