Align NamedKey and KeyCode more closely with the W3C specs (#4018)

By removing `NamedKey::Space` and rename "super" key to "meta".

- `NamedKey::Space` is not in the spec, and doesn't make sense to
  special-case. We use `Key::Character("")` instead..

  I've added an extra check on the Windows backend, to ensure that the code
  functionally works the same before and after. Whether that check is
  desirable or not can be figured out later.

- "super" is inconsistent with the W3C spec, and while it's arguably not the
  best name, it's worse that Winit is diverging and choosing a different name.

  List of renamings:
  - `KeyCode::SuperLeft` -> `KeyCode::MetaLeft`
  - `KeyCode::SuperRight` -> `KeyCode::MetaRight`
  - `KeyCode::Meta` -> `KeyCode::Super`
  - `NamedKey::Meta` -> `NamedKey::Super`
  - `NamedKey::Super` -> `NamedKey::Meta`
  - `ModifiersState::SUPER` -> `ModifiersState::META` (deprecated)
  - `ModifiersState::super_key` -> `ModifiersState::meta_key`
  - `ModifiersKeys::LSUPER` -> `ModifiersKeys::LMETA`
  - `ModifiersKeys::RSUPER` -> `ModifiersKeys::RMETA`

examples/application.rs

@@ -1163,7 +1163,16 @@ fn modifiers_to_string(mods: ModifiersState) -> String {
     let mut mods_line = String::new();
     // Always add + since it's printed as a part of the bindings.
     for (modifier, desc) in [
-        (ModifiersState::SUPER, "Super+"),
+        (
+            ModifiersState::META,
+            if cfg!(target_os = "windows") {
+                "Win+"
+            } else if cfg!(target_vendor = "apple") {
+                "Cmd+"
+            } else {
+                "Super+"
+            },
+        ),
         (ModifiersState::ALT, "Alt+"),
         (ModifiersState::CONTROL, "Ctrl+"),
         (ModifiersState::SHIFT, "Shift+"),
@@ -1264,10 +1273,10 @@ const KEY_BINDINGS: &[Binding<&'static str>] = &[
     Binding::new("Z", ModifiersState::CONTROL, Action::ToggleCursorVisibility),
     // K.
     Binding::new("K", ModifiersState::empty(), Action::SetTheme(None)),
-    Binding::new("K", ModifiersState::SUPER, Action::SetTheme(Some(Theme::Light))),
+    Binding::new("K", ModifiersState::META, Action::SetTheme(Some(Theme::Light))),
     Binding::new("K", ModifiersState::CONTROL, Action::SetTheme(Some(Theme::Dark))),
     #[cfg(macos_platform)]
-    Binding::new("T", ModifiersState::SUPER, Action::CreateNewTab),
+    Binding::new("T", ModifiersState::META, Action::CreateNewTab),
     #[cfg(macos_platform)]
     Binding::new("O", ModifiersState::CONTROL, Action::CycleOptionAsAlt),
     Binding::new("S", ModifiersState::CONTROL, Action::Message),