Add text_color helper for tab contents (#14737)

This PR adds a `text_color` method to `TabContentParams` to more easily
compute the text color to be used for tab contents.

This consolidates a number of conditionals that were scattered all over
the place to give us a singular source of truth for these colors.

Release Notes:

- N/A

crates/image_viewer/src/image_viewer.rs

@@ -80,11 +80,7 @@ impl Item for ImageView {
             .to_string();
         Label::new(title)
             .single_line()
-            .color(if params.selected {
-                Color::Default
-            } else {
-                Color::Muted
-            })
+            .color(params.text_color())
             .italic(params.preview)
             .into_any_element()
     }