Settings: a settings window, its sections, a search, and what fills them

SettingsLayout lists sections down the side and shows the open one, the
list going above the page in a narrow box; SettingsSection and
SettingsRow title and hold settings, a row's control wrapping under its
name. SettingsSearch finds a setting by its name or words and opens its
section. ThemeSelector, AccentColorPicker, FontSizeControl and
DensitySelector set the look; KeyboardShortcutsList finds shortcuts by
every word; ProxySettings, PrivacySettings, NotificationSettings,
StartupSettings, StorageSettings with its clear, ImportExportSettings,
ResetToDefault, FeatureFlags and DeveloperModeToggle fill the rest.
Every handler a control needs is required and fails loud when missing.
ColorPalette wraps its swatches in a row capped at its columns, so it
keeps their size in a box sized by its content, and its swatches carry
the palette's id. ThemeSwitcher points here: src/theme sits under the
chapters that draw with buttons.

src/settings/mod.rs

@@ -0,0 +1,24 @@
+mod advanced;
+mod appearance;
+mod network;
+mod preferences;
+mod search;
+mod section;
+mod shortcuts;
+mod storage;
+
+#[cfg(all(test, feature = "test-support"))]
+mod tests;
+
+pub use advanced::{DeveloperModeToggle, FeatureFlags, Flag, Stage};
+pub use appearance::{
+    AccentColorPicker, Appearance, DensitySelector, FontSizeControl, ThemeSelector,
+};
+pub use network::{Proxy, ProxySettings, manual};
+pub use preferences::{
+    Notices, NotificationSettings, Privacy, PrivacySettings, Startup, StartupSettings,
+};
+pub use search::{SettingEntry, SettingsSearch, matching};
+pub use section::{SettingsLayout, SettingsRow, SettingsSection};
+pub use shortcuts::{KeyboardShortcutsList, Shortcut};
+pub use storage::{Changed, ImportExportSettings, ResetToDefault, StorageSettings, Store};