scheme: Add user-defined schemes with dynamic fallback

[XLhinares]

This PR adds a new scheme: manual.

Why

The dynamic scheme works nicely with some wallpapers, and with others I like to use my own scheme. However, if I use a custom scheme, it stays on when I change my wallpaper, which is not ideal either.

Additionally, overwriting the scheme file with a posthook script is not a good option since other scripts watching wallpaper change will use the scheme before it is overwritten.

How

The manual scheme checks if the user has a custom scheme file for the current wallpaper at the path ~/.config/caelestia/schemes/<wallpaper_name>.txt.
If that file exists, then it is used as the theme; otherwise, the dynamic scheme is used.

Notes

• The user needs to create ~/.config/caelestia/schemes/ to put their custom schemes.
• The <wallpaper_name> also contains the file extension (so the scheme name will end up like my_wallpaper.png.txt)
• If the user has two wallpapers with the same name and extension, they will use the same manual scheme, but I don't think that's an issue.
• Manual scheme has the same flavors as dynamic, but doesn't directly use them (they are used when the scheme falls back to dynamic)

[XLhinares]

I changed the way the schemes are looked up: the cli now use a json file (~/.config/caelestia/manual-schemes.json) to match wallpaper name and scheme path.

Advantages:

• This allows user to reuse themes more easily
• The config folder is less clutered (one json vs a dir full of schemes)
• The user can store schemes wherever

Example json:

{
  "978293.png": "~/.config/caelestia/schemes/black-gold-scheme.txt",
  "abstract-black-gold.png": "~/.config/caelestia/schemes/black-gold-scheme.txt",
  "abstract-black-purple.png": "~/.config/caelestia/schemes/abstract-black-purple.png.txt"
}

[XLhinares]

Last commit introduced regex matching.
The manual scheme strategy is now:

• If the exact wallpaper file name is a key in the json, use the matching scheme,
• Otherwise, if the wallpaper path matches a regex pattern in the json, use that scheme,
• If neither match, fallback to the dynamic scheme

Example json:

{
  "978293.png": "~/.config/caelestia/schemes/black-gold-scheme.txt",
  "abstract-black-gold.png": "~/.config/caelestia/schemes/black-gold-scheme.txt",
  "abstract-black-purple.png": "~/.config/caelestia/schemes/abstract-black-purple.png.txt",
  "Wallpapers/dark/": "~/Downloads/dark-scheme.txt",
  ".*": "~/.config/caelestia/schemes/default.txt"
}

Note:

• if several regex match the wallpaper path, the first one is used.