settings_ui: Add UI and buffer font family controls (#15124)

This PR adds settings controls for changing the UI and buffer font
families.

Release Notes:

- N/A

crates/theme/src/font_family_cache.rs

@@ -0,0 +1,52 @@
+use std::sync::Arc;
+use std::time::Instant;
+
+use gpui::{AppContext, Global, ReadGlobal, SharedString};
+use parking_lot::RwLock;
+
+#[derive(Default)]
+struct FontFamilyCacheState {
+    loaded_at: Option<Instant>,
+    font_families: Vec<SharedString>,
+}
+
+/// A cache for the list of font families.
+///
+/// Listing the available font families from the text system is expensive,
+/// so we do it once and then use the cached values each render.
+#[derive(Default)]
+pub struct FontFamilyCache {
+    state: RwLock<FontFamilyCacheState>,
+}
+
+#[derive(Default)]
+struct GlobalFontFamilyCache(Arc<FontFamilyCache>);
+
+impl Global for GlobalFontFamilyCache {}
+
+impl FontFamilyCache {
+    pub fn init_global(cx: &mut AppContext) {
+        cx.default_global::<GlobalFontFamilyCache>();
+    }
+
+    pub fn global(cx: &AppContext) -> Arc<Self> {
+        GlobalFontFamilyCache::global(cx).0.clone()
+    }
+
+    pub fn list_font_families(&self, cx: &AppContext) -> Vec<SharedString> {
+        if self.state.read().loaded_at.is_some() {
+            return self.state.read().font_families.clone();
+        }
+
+        let mut lock = self.state.write();
+        lock.font_families = cx
+            .text_system()
+            .all_font_names()
+            .into_iter()
+            .map(SharedString::from)
+            .collect();
+        lock.loaded_at = Some(Instant::now());
+
+        lock.font_families.clone()
+    }
+}

crates/theme/src/theme.rs

@@ -8,6 +8,7 @@
 
 mod default_colors;
 mod default_theme;
+mod font_family_cache;
 mod one_themes;
 pub mod prelude;
 mod registry;
@@ -21,6 +22,7 @@ use std::sync::Arc;
 use ::settings::{Settings, SettingsStore};
 pub use default_colors::*;
 pub use default_theme::*;
+pub use font_family_cache::*;
 pub use registry::*;
 pub use scale::*;
 pub use schema::*;
@@ -82,6 +84,7 @@ pub fn init(themes_to_load: LoadThemes, cx: &mut AppContext) {
     }
 
     ThemeSettings::register(cx);
+    FontFamilyCache::init_global(cx);
 
     let mut prev_buffer_font_size = ThemeSettings::get_global(cx).buffer_font_size;
     cx.observe_global::<SettingsStore>(move |cx| {