New rnote file format, better compression and serialization, atomic file saving

[anesthetice]

I've extended this pull request with #1150 to share code in common, and #1170 as they were in conflict.

The overall goal of this PR is to improve the handling of Rnote save files, this is accomplished mainly in three ways:

1. A more flexible, backwards-compatible Rnote file format
   (fixes New file format (stabilization) #1173)
2. Better default compression and serialization (zstd + bitcode instead of gzip + json)
3. Atomic saves
   (fixes More resilient file saving by a two-stage file save process #1128)

1 - the Rnote file format

With regards to what was discussed on this issue here #1173:

• backwards compatible

• this format supports (and can be extended to support)

  • multiple compression methods
  • multiple serialization method
  • additional metadata

• the file's version and metadata are kept separate from the (potentially) compressed data

• still unsure as to how well this new format would support partial file loading, but I believe it could be possible by adding extra header info (such as an array containing the start of each sub-bloc in the body of the file)

2 - compression and serialization

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

In addition to the default compression and serialization method changes, users can also select their desired compression level, from "Very Low" to "Very High" in the "Document" section of the settings, Rnote re-uses the previously selected compression level for new files, and as for existent files, maintains their compression level unless explicitly changed by the user.

For debugging purposes, the serialization and compression methods can be set to non-standard variants, and can be maintained even after re-saving the file by setting method_lock to true

These features are achieved by using save preferences (EngineSnapshot <-> Engine <-> EngineConfig)

3 - atomic saves

Current

• the save file is simply overwritten

Propsal:

• write to temporary file
• check temporary file
• replace save file with temporary one

4 - other cool stuff

rnote-cli mutate

Ever wanted to further compress Rnote files you don't often use but still wish to have direct access to?
rnote-cli mutate --compression-method zstd --compression-level 19 --serialization-method bitcode $(find . -name "*.rnote" 2>/dev/null)

Want an uncompressed Rnote file encoded in json?
rnote-cli mutate --compression-method none --serialization-method json --lock file.rnote

5 - pinboard

• json:
  • uncompressed: 46 MB
  • compressed: 5.5 MB
• cbor:
  • uncompressed: 39MB
  • compressed: 6.8 MB
• bincode:
  • uncompressed: 20.9 MB
  • compressed: 6.4 MB
• postcard:
  • uncompressed: 18.6 MB
  • compressed: 6.3 MB
• toml:
  • no
• bitcode (using serialize and not encode):
  • uncompressed: 17.4 MB
  • compressed: 4.9 MB

Deserialized from Json in 0.822; Serialized to 46.201438 MB, in 0.518
Deserialized from Cbor in 0.229; Serialized to 39.910991 MB, in 0.073
Deserialized from Bincode in 0.027; Serialized to 20.993607 MB, in 0.016
Deserialized from Toml in 5.255; Serialized to 112.392787 MB, in 7.203
Deserialized from Bitcode in 0.020; Serialized to 17.432074 MB, in 0.028
Deserialized from Pot in 0.120; Serialized to 28.007348 MB, in 0.049
Deserialized from Postcard in 0.017; Serialized to 18.668699 MB, in 0.017

[anesthetice]

demo 1 - backwards compatibility, size ratio, persistent compression level

rnote_demo_1.mp4

demo 2 - rnote-cli mutate (atomic file saving is also noticeable)

rnote_demo_2.mp4

[anesthetice]

I went through all of my code yesterday, I'd say this is ready for review, still open to suggestions or improvements of course