Anim: Action editor, show slot user count if not equal to 1

In the Action editor, show the number of users for each slot.

- 1 user (the expected case): not shown
- multiple users: number is shown between parentheses, f.e. `(3)`
- none: shown as `(unassigned)` as I think that's clearer than `(0)`.

This PR doesn't technically depend on !122500, but having that landed
first will make testing this one quite a bit easier.

Pull Request: https://projects.blender.org/blender/blender/pulls/124337

source/blender/editors/animation/anim_channels_defines.cc

@@ -1369,7 +1369,22 @@ static void acf_action_slot_name(bAnimListElem *ale, char *r_name)
     return;
   }
 
-  BLI_strncpy(r_name, slot->name_without_prefix().c_str(), ANIM_CHAN_NAME_SIZE);
+  BLI_assert(ale->bmain);
+  const int num_users = slot->users(*ale->bmain).size();
+  const char *display_name = slot->name_without_prefix().c_str();
+
+  BLI_assert(num_users >= 0);
+  switch (num_users) {
+    case 0:
+      BLI_snprintf(r_name, ANIM_CHAN_NAME_SIZE, "%s (unassigned)", display_name);
+      break;
+    case 1:
+      BLI_strncpy(r_name, display_name, ANIM_CHAN_NAME_SIZE);
+      break;
+    default:
+      BLI_snprintf(r_name, ANIM_CHAN_NAME_SIZE, "%s (%d)", display_name, num_users);
+      break;
+  }
 }
 static bool acf_action_slot_name_prop(bAnimListElem *ale, PointerRNA *r_ptr, PropertyRNA **r_prop)
 {