zstd with parallelism instead of gzip

[anesthetice]

https://github.com/anesthetice/rnote-compression-benchmarking/

From the compression methods tested, zstd seems to be a better choice for the rnote file format:

• Better compression ratio
• Faster reads + writes
• Simple multi-threading

[anesthetice]

Implementation went smoother than what I expected, rnote can still decompress gzip files and will compress them using zstd the next time the file is saved. One thing left to discuss I think is, should we expose the compression level to users?

[anesthetice]

Here's a rough before and after comparison of a couple files (different to the ones used for rnote-comp-benchmarking)

Before:

After:

[RayJW]

Amazing work, that is exactly what I was talking about. Thanks, for looking into this! Looks like it's definitely worth considering. :)

[anesthetice]

converted this to a draft, to focus on an updated file format

[anesthetice]

closing this to de-clutter, see #1177