Add staged checkboxes to multibuffer headers (#24308)

Co-authored-by: Mikayla <mikayla@zed.dev>

Release Notes:

- N/A

---------

Co-authored-by: Mikayla <mikayla@zed.dev>

crates/ui/src/traits/toggleable.rs

@@ -58,12 +58,12 @@ impl From<bool> for ToggleState {
     }
 }
 
-// impl From<Option<bool>> for ToggleState {
-//     fn from(selected: Option<bool>) -> Self {
-//         match selected {
-//             Some(true) => Self::Selected,
-//             Some(false) => Self::Unselected,
-//             None => Self::Unselected,
-//         }
-//     }
-// }
+impl From<Option<bool>> for ToggleState {
+    fn from(selected: Option<bool>) -> Self {
+        match selected {
+            Some(true) => Self::Selected,
+            Some(false) => Self::Unselected,
+            None => Self::Indeterminate,
+        }
+    }
+}