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.

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