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/networking.rs

@@ -1,43 +0,0 @@
-use super::{Page, SubPage};
-
-#[derive(Clone, Copy, Debug, Eq, PartialEq)]
-pub enum NetworkingPage {
-    Wired,
-    OnlineAccounts,
-}
-
-impl SubPage for NetworkingPage {
-    //TODO: translate
-    fn title(&self) -> &'static str {
-        use NetworkingPage::*;
-        match self {
-            Wired => "Wired",
-            OnlineAccounts => "Online Accounts",
-        }
-    }
-
-    //TODO: translate
-    fn description(&self) -> &'static str {
-        use NetworkingPage::*;
-        match self {
-            Wired => "Wired connection, connection profiles",
-            OnlineAccounts => "Add accounts, IMAP and SMTP, enterprise logins",
-        }
-    }
-
-    fn icon_name(&self) -> &'static str {
-        use NetworkingPage::*;
-        match self {
-            Wired => "network-workgroup-symbolic",
-            OnlineAccounts => "goa-panel-symbolic", //TODO: new icon
-        }
-    }
-
-    fn parent_page(&self) -> Page {
-        Page::Networking(None)
-    }
-
-    fn into_page(self) -> Page {
-        Page::Networking(Some(self))
-    }
-}