src: Fix the API of KeymapManager::add_user_mode()

Cppcheck produces the following warning:

```
keymap_manager.hh:54:37: performance: Function parameter 'user_mode_name' should be passed by const reference.
```

Fixes #4340

src/keymap_manager.hh

@@ -51,7 +51,7 @@ public:
             return m_parent->user_modes();
         return m_user_modes;
     }
-    void add_user_mode(const String user_mode_name);
+    void add_user_mode(String user_mode_name);
 
 private:
     KeymapManager()