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/system_and_accounts.rs

@@ -1,94 +0,0 @@
-use cosmic::{
-    iced::widget::{horizontal_space, row, text},
-    iced::Length,
-    widget::{icon, list_column, settings},
-    Element,
-};
-
-use super::{Message, Page, SubPage, Window};
-
-#[derive(Default)]
-pub struct State {}
-
-#[derive(Clone, Copy, Debug, Eq, PartialEq)]
-pub enum SystemAndAccountsPage {
-    Users,
-    About,
-    Firmware,
-}
-
-impl SubPage for SystemAndAccountsPage {
-    //TODO: translate
-    fn title(&self) -> &'static str {
-        use SystemAndAccountsPage::*;
-        match self {
-            Users => "Users",
-            About => "About",
-            Firmware => "Firmware",
-        }
-    }
-
-    //TODO: translate
-    fn description(&self) -> &'static str {
-        use SystemAndAccountsPage::*;
-        match self {
-            Users => "Authentication and login, lock screen.",
-            About => "Device name, hardware information, operating system defaults.",
-            Firmware => "Firmware details.",
-        }
-    }
-
-    fn icon_name(&self) -> &'static str {
-        use SystemAndAccountsPage::*;
-        match self {
-            Users => "system-users-symbolic",
-            About => "help-about-symbolic",
-            Firmware => "firmware-manager-symbolic",
-        }
-    }
-
-    fn parent_page(&self) -> Page {
-        Page::SystemAndAccounts(None)
-    }
-
-    fn into_page(self) -> Page {
-        Page::SystemAndAccounts(Some(self))
-    }
-}
-
-impl State {
-    pub(super) fn view<'a>(&'a self, window: &'a Window) -> Element<'a, Message> {
-        settings::view_column(vec![
-            window.parent_page_button(SystemAndAccountsPage::About),
-            row!(
-                horizontal_space(Length::Fill),
-                icon("distributor-logo", 78),
-                horizontal_space(Length::Fill),
-            )
-            .into(),
-            list_column()
-                .add(settings::item("Device name", text("TODO")))
-                .into(),
-            settings::view_section("Hardware")
-                .add(settings::item("Hardware model", text("TODO")))
-                .add(settings::item("Memory", text("TODO")))
-                .add(settings::item("Processor", text("TODO")))
-                .add(settings::item("Graphics", text("TODO")))
-                .add(settings::item("Disk Capacity", text("TODO")))
-                .into(),
-            settings::view_section("Operating System")
-                .add(settings::item("Operating system", text("TODO")))
-                .add(settings::item(
-                    "Operating system architecture",
-                    text("TODO"),
-                ))
-                .add(settings::item("Desktop environment", text("TODO")))
-                .add(settings::item("Windowing system", text("TODO")))
-                .into(),
-            settings::view_section("Related settings")
-                .add(settings::item("Get support", text("TODO")))
-                .into(),
-        ])
-        .into()
-    }
-}