Extract title_bar crate (#13597)

This PR extracts a singular title bar (`title_bar::TitleBar`) from
`ui::TitleBar` and
`collab_ui::collab_titlebar_item::CollabTitlebarItem`.

This is a first step towards organizing title bar things into one place,
and standardizing platform titlebar/window control implementations.

Release Notes:

- N/A

crates/assistant/src/prompt_library.rs

@@ -34,7 +34,7 @@ use std::{
 use theme::ThemeSettings;
 use ui::{
     div, prelude::*, IconButtonShape, ListItem, ListItemSpacing, ParentElement, Render,
-    SharedString, Styled, TitleBar, Tooltip, ViewContext, VisualContext,
+    SharedString, Styled, Tooltip, ViewContext, VisualContext,
 };
 use util::{ResultExt, TryFutureExt};
 use uuid::Uuid;
@@ -751,7 +751,7 @@ impl PromptLibrary {
             .child(
                 h_flex()
                     .p(Spacing::Small.rems(cx))
-                    .h(TitleBar::height(cx))
+                    .h_9()
                     .w_full()
                     .flex_none()
                     .justify_end()