Improve actions macros (#3292)

- `actions!` now uses `#[action]` on each struct to reduce duplication.
- The `#[action]` macro now works on unit structs.
- Renamed `menu::unused` to `menu::init` and added more explanation in
comments.

Release Notes:

- N/A

crates/gpui2/src/action.rs

@@ -176,8 +176,7 @@ macro_rules! actions {
     () => {};
 
     ( $name:ident ) => {
-        #[gpui::register_action]
-        #[derive(::std::clone::Clone, ::std::default::Default, ::std::fmt::Debug, ::std::cmp::PartialEq, $crate::serde::Deserialize)]
+        #[gpui::action]
         pub struct $name;
     };