Keyboard shortcut helpers (#2202)

* eframe web: Add WebInfo::user_agent

* Deprecate `Modifier::ALT_SHIFT`

* Add code for formatting Modifiers and Key

* Add type KeyboardShortcut

* Code cleanup

* Add Context::os/set_os to query/set what OS egui believes it is on

* Add Fonts::has_glyph(s)

* Add helper function for formatting keyboard shortcuts

* Faster code

* Add way to set a shortcut text on menu buttons

* Cleanup

* format_keyboard_shortcut -> format_shortcut

* Add TODO about supporting more keyboard sumbols

* Modifiers::plus

* Use the new keyboard shortcuts in emark editor demo

* Explain why ALT+SHIFT is a bad modifier combo

* Fix doctest

crates/eframe/src/web/backend.rs

@@ -87,6 +87,10 @@ impl IsDestroyed {
 
 // ----------------------------------------------------------------------------
 
+fn user_agent() -> Option<String> {
+    web_sys::window()?.navigator().user_agent().ok()
+}
+
 fn web_location() -> epi::Location {
     let location = web_sys::window().unwrap().location();
 
@@ -198,6 +202,7 @@ impl AppRunner {
 
         let info = epi::IntegrationInfo {
             web_info: epi::WebInfo {
+                user_agent: user_agent().unwrap_or_default(),
                 location: web_location(),
             },
             system_theme,
@@ -207,6 +212,9 @@ impl AppRunner {
         let storage = LocalStorage::default();
 
         let egui_ctx = egui::Context::default();
+        egui_ctx.set_os(egui::os::OperatingSystem::from_user_agent(
+            &user_agent().unwrap_or_default(),
+        ));
         load_memory(&egui_ctx);
 
         let theme = system_theme.unwrap_or(web_options.default_theme);