system/icons: improve inherits, allow png icons, fixup pathing, add caching

[vaxerski]

Various improvements, fixes #43

cc @Dregu please check this

[Dregu]

• This does now fallback to the hicolor theme inherited in the default theme explicitly, but there's still supposed to be a hardcoded fallback to hicolor too
• It's finding more icons now, but mostly SVG
• PNG seems to only work from /pixmaps, not /icons. e.g. fuzzel has a lot more icons still
• The few PNG that do work look pretty crusty in hyprlauncher, maybe expected cause resized

[vaxerski]

1. done
2. fixed
3. fixed, but see note
4. expected

Note: on my system I have ~/.local/share/icons/hicolor, but it does not have an index.theme file, so ht ignores it. Fuzzel doesn't... I think?

[Dregu]

~/.local/share/icons/hicolor

Yeah these icons are usually from when steam/lutris/$gamelauncher makes a desktop shortcut. They do work in fuzzel, not hyprlauncher.

Everything found under $themename anywhere should be combined under the common $themename, and anything found in higher priority directories should override anything in the lower ones. e.g. user should be allowed to only create a ~/.local/share/icons/hicolor/scalable/foot.svg and nothing else to override a single apps icon from /usr/share/icons/hicolor/. Or add a bunch of new ones. It's probably actually way more common to just put a bunch of extra icons in .local instead of a complete icon theme with the index, but overriding the index file should also be possible, technically.

For me this has like >99% of icons now, just the games are missing.

Oh and I just remembered that Icon=/absolute/path.ext is also a thing that probably doesn't work, but that's also the simplest feature ever to add if it doesn't exist yet. Some (nix) systems might use absolute paths exclusively I think.

[vaxerski]

this can be improved later, I think it's good enough