Keyboard shortcut helpers

[emilk]

This adds egui::KeyboardShortcut which is Modifiers + Key.

You can check it with input.consume_shortcut.
You can format it to text using Context::format_shortcut.
You can set that text on buttons (e.g. for menus) with Button::shortcut_text.

If you use Modifiers::COMMAND, egui will check it against the Mac Command key (⌘) on Mac, but check against Ctrl on other platforms. This means how the shortcut is formatted depends on the OS you are running on. This is difficult to know when compiling for wasm32, so egui now also has Context::set_os and Context::os to set/get that. set_os is meant for egui integrartions. eframe will guess the OS from the User-Agent string when on web. On native it will use target_os.

Bonus 1: You can now ask eframe about the User-Agent string when on web using frame.info().web_info.user_agent.
Bonus 2: You can now query if epaint/egui can show some characters by using Fonts::has_glyph(s).

TODO:

• PR description
• Changelogs
• Menu button support