Differentiate among tabs with the same name

This commit introduces a new, optional `Item::tab_description` method
that lets implementers define a description for the tab with a certain
`detail`. When two or more tabs match the same description, we will
increase the `detail` until tabs don't match anymore or increasing the
`detail` doesn't disambiguate tabs any further.

As soon as we find a valid `detail` that disambiguates tabs enough, we
will pass it to `Item::tab_content`. In `Editor`, this is implemented by
showing more and more of the path's suffix as `detail` is increased.

crates/theme/src/theme.rs

@@ -93,6 +93,7 @@ pub struct Tab {
     pub container: ContainerStyle,
     #[serde(flatten)]
     pub label: LabelStyle,
+    pub description: ContainedText,
     pub spacing: f32,
     pub icon_width: f32,
     pub icon_close: Color,