Reimplemented NavigationBar

- Navigation Bar was reimplemented to support sections and pages.
- Created new widget called separator, a horizontal rule with the COSMIC
  theme.

src/widget/separator.rs

@@ -0,0 +1,7 @@
+#[macro_export]
+macro_rules! separator {
+    ($size:expr) => {
+        $crate::iced::widget::horizontal_rule($size)
+            .style(theme::Rule::Custom($crate::widget::separator_style))
+    };
+}