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/title_bar/src/platforms/platform_mac.rs

@@ -0,0 +1,6 @@
+/// Use pixels here instead of a rem-based size because the macOS traffic
+/// lights are a static size, and don't scale with the rest of the UI.
+///
+/// Magic number: There is one extra pixel of padding on the left side due to
+/// the 1px border around the window on macOS apps.
+pub const TRAFFIC_LIGHT_PADDING: f32 = 71.;