feat(Utils): blurhash

[GeopJr]

fix: #487

This is a pure Vala blurhash decoding implementation inspired by https://github.com/woltapp/blurhash and https://github.com/mad-gooze/fast-blurhash/.

FAQ:

• Q: Why not use the 'official' library?
• A: It's unoptimized (for example, it calculates sizeFlag, numX, numY and size twice, once for the validation check and then for the decoding again). Additionally, by writing it in pure Vala it makes use of Glib (and lets Vala handle the memory).
• Q: Why does it not match the 'official' library's results 1:1?
• A: It uses parts of fast-blurhash's algorithm, which is (or at least should be) faster but has slightly different results.

TODO:

• unit tests
• actually use them
• move the pixbuf -> paintable part into its own function?