Make a macro for less boilerplate when moving variables (#12182)

Also: 
- Simplify open listener implementation
- Add set_global API to global traits

Release Notes:

- N/A

crates/gpui/src/global.rs

@@ -49,6 +49,11 @@ pub trait UpdateGlobal {
     where
         C: BorrowAppContext,
         F: FnOnce(&mut Self, &mut C) -> R;
+
+    /// Set the global instance of the implementing type.
+    fn set_global<C>(cx: &mut C, global: Self)
+    where
+        C: BorrowAppContext;
 }
 
 impl<T: Global> UpdateGlobal for T {
@@ -59,4 +64,11 @@ impl<T: Global> UpdateGlobal for T {
     {
         cx.update_global(update)
     }
+
+    fn set_global<C>(cx: &mut C, global: Self)
+    where
+        C: BorrowAppContext,
+    {
+        cx.set_global(global)
+    }
 }