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/ui/src/styles/platform.rs

@@ -0,0 +1,25 @@
+/// The platform style to use when rendering UI.
+///
+/// This can be used to abstract over platform differences.
+#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Copy)]
+pub enum PlatformStyle {
+    /// Display in macOS style.
+    Mac,
+    /// Display in Linux style.
+    Linux,
+    /// Display in Windows style.
+    Windows,
+}
+
+impl PlatformStyle {
+    /// Returns the [`PlatformStyle`] for the current platform.
+    pub const fn platform() -> Self {
+        if cfg!(target_os = "linux") {
+            Self::Linux
+        } else if cfg!(target_os = "windows") {
+            Self::Windows
+        } else {
+            Self::Mac
+        }
+    }
+}