cosmic: Provide CosmicConfigEntry impl, use _ in property names

It seems to derive `CosmicConfigEntry`, we need to use `_` in property
names, because `_` isn't mapped to `-` and there doesn't seem to be a
mechanism like `#[serde(rename)]`.

It seems good to be consistent anyway. So this seems good to change,
unless we really like using `-` for names on disk, then cosmic-panel
needs to be changed.

`CosmicConfigEntry` also requires `PartialEq`, which is easy enough to
add.

This will break existing input settings, which will have to be
re-applied with the new locations. Cosmic-settings also need to be
updated for this.

cosmic-comp-config/Cargo.toml

@@ -4,5 +4,6 @@ version = "0.1.0"
 edition = "2021"
 
 [dependencies]
+cosmic-config = { git = "https://github.com/pop-os/libcosmic/" }
 input = "0.8.3"
 serde = { version = "1", features = ["derive"] }