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/window/bluetooth.rs

@@ -0,0 +1,43 @@
+use super::{Page, Window};
+use cosmic::{
+    iced::widget::{column, text},
+    widget::{list_column, settings, toggler},
+    Element,
+};
+
+#[derive(Clone, Copy, Debug)]
+pub enum Message {
+    Enable(bool),
+}
+
+#[derive(Default)]
+pub struct State {
+    enabled: bool,
+}
+
+impl State {
+    pub(super) fn update(&mut self, message: Message) {
+        match message {
+            Message::Enable(value) => self.enabled = value,
+        }
+    }
+
+    pub(super) fn view<'a>(&'a self, window: &'a Window) -> Element<'a, Message> {
+        settings::view_column(vec![
+            window.page_title(Page::Bluetooth),
+            column!(
+                list_column().add(settings::item(
+                    "Bluetooth",
+                    toggler(None, self.enabled, Message::Enable)
+                )),
+                text("Now visible as \"TODO\", just kidding")
+            )
+            .spacing(8)
+            .into(),
+            settings::view_section("Devices")
+                .add(settings::item("No devices found", text("")))
+                .into(),
+        ])
+        .into()
+    }
+}