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/src/window/bluetooth.rs

@@ -1,43 +0,0 @@
-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()
-    }
-}