Fixed sidebar responsiveness

- Created new widget `scrollbar` which can adopt COSMIC styling.
- Increased sidebar icon size by 4 pixels to improve visibility.
- Added padding inside the navigation bar to prevent collision with the scrollbar.
- Increased the max width of the condensed navbar for the scrollbar.

src/widget/scrollbar.rs

@@ -0,0 +1,8 @@
+#[macro_export]
+macro_rules! scrollbar {
+    ($x:expr) => (
+        $crate::iced::widget::scrollable($x)
+            .scrollbar_width(8)
+            .scroller_width(8)
+    );
+}