Organize settings dialog code for extensability and add menu transitions to dialog. (#455)

widget/settings/settingsPages.ts

@@ -0,0 +1,9 @@
+import { SettingsMenu } from './pages/config/index';
+import { ThemesMenu } from './pages/theme/index';
+
+export const settingsPages = {
+    Configuration: SettingsMenu(),
+    Theming: ThemesMenu(),
+} as const;
+
+export type SettingsPage = keyof typeof settingsPages;