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/editor.rs

@@ -1,79 +0,0 @@
-use apply::Apply;
-use cosmic::iced::widget::{horizontal_space, row, scrollable};
-use cosmic::iced::Length;
-use cosmic::iced_winit::Alignment;
-use cosmic::widget::{button, segmented_button, view_switcher};
-use cosmic::{theme, Element};
-use slotmap::Key;
-
-#[derive(Clone, Copy, Debug)]
-pub enum Message {
-    Activate(segmented_button::Entity),
-    AddNew,
-    Close(segmented_button::Entity),
-}
-
-pub struct State {
-    pub pages: segmented_button::SingleSelectModel,
-}
-
-impl Default for State {
-    fn default() -> Self {
-        let mut state = Self {
-            pages: segmented_button::Model::default(),
-        };
-
-        let id = state.tab_add_new();
-        state.pages.activate(id);
-
-        state
-    }
-}
-
-impl State {
-    pub(super) fn update(&mut self, message: Message) {
-        match message {
-            Message::Activate(id) => self.pages.activate(id),
-            Message::AddNew => {
-                self.tab_add_new();
-            }
-            Message::Close(id) => self.tab_close(id),
-        }
-    }
-
-    pub fn tab_add_new(&mut self) -> segmented_button::Entity {
-        let id = self.pages.insert().closable().id();
-
-        self.pages
-            .text_set(id, format!("Tab {}", id.data().as_ffi() & 0xffff_ffff));
-
-        id
-    }
-
-    pub fn tab_close(&mut self, id: segmented_button::Entity) {
-        if self.pages.is_active(id) {
-            if let Some(pos) = self.pages.position(id) {
-                let next = if pos == 0 { pos + 1 } else { pos - 1 };
-                self.pages.activate_position(next);
-            }
-        }
-
-        self.pages.remove(id);
-    }
-
-    pub(super) fn view<'a>(&'a self, window: &'a super::Window) -> Element<'a, Message> {
-        let tabs = view_switcher::horizontal(&self.pages)
-            .show_close_icon_on_hover(true)
-            .on_activate(Message::Activate)
-            .on_close(Message::Close)
-            .width(Length::Shrink);
-
-        let new_tab_button = button(theme::Button::Text)
-            .icon(theme::Svg::Symbolic, "tab-new-symbolic", 20)
-            .on_press(Message::AddNew);
-
-        let tab_header = row!(tabs, new_tab_button).align_items(Alignment::Center);
-
-        row!(tab_header, horizontal_space(Length::Fill)).into()
-    }
-}