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.

styles/src/styleTree/workspace.ts

@@ -27,6 +27,10 @@ export default function workspace(theme: Theme) {
       left: 8,
       right: 8,
     },
+    description: {
+      margin: { left: 6, top: 1 },
+      ...text(theme, "sans", "muted", { size: "2xs" })
+    }
   };
 
   const activeTab = {