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/time_and_language.rs

@@ -0,0 +1,43 @@
+use super::{Page, SubPage};
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum TimeAndLanguagePage {
+    DateAndTime,
+    RegionAndLanguage,
+}
+
+impl SubPage for TimeAndLanguagePage {
+    //TODO: translate
+    fn title(&self) -> &'static str {
+        use TimeAndLanguagePage::*;
+        match self {
+            DateAndTime => "Date & Time",
+            RegionAndLanguage => "Region & Language",
+        }
+    }
+
+    //TODO: translate
+    fn description(&self) -> &'static str {
+        use TimeAndLanguagePage::*;
+        match self {
+            DateAndTime => "Time zone, automatic clock settings, and some time formatting.",
+            RegionAndLanguage => "Format dates, times, and numbers based on your region",
+        }
+    }
+
+    fn icon_name(&self) -> &'static str {
+        use TimeAndLanguagePage::*;
+        match self {
+            DateAndTime => "preferences-system-time-symbolic",
+            RegionAndLanguage => "preferences-desktop-locale-symbolic",
+        }
+    }
+
+    fn parent_page(&self) -> Page {
+        Page::TimeAndLanguage(None)
+    }
+
+    fn into_page(self) -> Page {
+        Page::TimeAndLanguage(Some(self))
+    }
+}