Make example runnable as example

Trying to run examples as usual (and as described in the README) fails:

    > cargo run --release --example cosmic
    error: no example target named `cosmic`.

The examples themselves are written as separate crates, and are
therefore not usable as examples (they also won't build via `cargo build
--examples`).

This commit makes the example fit the style that cargo can understand.
That is:

- Don't declare the example as a separate crate.
- Remove the nested `src` inside the example directory.
- Put example-only dependencies in dev-dependencies

No rust code was modified, files were only moved.

Cargo.toml

@@ -74,10 +74,12 @@ optional = true
 path = "iced/glow"
 optional = true
 
+[dev-dependencies]
+once_cell = "1.15"
+
 [workspace]
 members = [
   "cosmic-config",
-  "examples/*",
 ]
 exclude = [
   "iced",