ui: Refine TitleBar component (#9415)

This PR continues the refinements to the `TitleBar` component.

Here are the notable changes:

- `KeyBindingDisplay` and `PlatformStyle` have been unified into a
single `PlatformStyle`.
- This provides us a consistent way for adapting UI to different
platform styles.
- `PlatformTitlebar` has been renamed to `TitleBar`.
  - The `Platform` prefix was irrelevant.
- The Windows window controls have been factored out into a separate
module and have been componentized.

<img width="1283" alt="Screenshot 2024-03-15 at 3 34 38 PM"
src="https://github.com/zed-industries/zed/assets/1486634/07da391f-828b-48bf-8849-58863f4ccce7">

> I'm missing the Segoe Fluent Icons font, so that's why the aren't
rendering properly.

Release Notes:

- N/A

crates/workspace/src/workspace.rs

@@ -4809,7 +4809,7 @@ impl Element for DisconnectedOverlay {
             .bg(background)
             .absolute()
             .left_0()
-            .top(ui::titlebar_height(cx))
+            .top(ui::TitleBar::height(cx))
             .size_full()
             .flex()
             .items_center()