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.

examples/cosmic/main.rs

@@ -0,0 +1,14 @@
+// Copyright 2022 System76 <info@system76.com>
+// SPDX-License-Identifier: MPL-2.0
+
+use cosmic::{iced::Application, settings};
+
+mod window;
+pub use window::*;
+
+pub fn main() -> cosmic::iced::Result {
+    settings::set_default_icon_theme("Pop");
+    let mut settings = settings();
+    settings.window.min_size = Some((600, 300));
+    Window::run(settings)
+}