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/epi.rs

@@ -767,6 +767,9 @@ impl Frame {
 #[derive(Clone, Debug)]
 #[cfg(target_arch = "wasm32")]
 pub struct WebInfo {
+    /// The browser user agent.
+    pub user_agent: String,
+
     /// Information about the URL.
     pub location: Location,
 }