Renames signal for consistency

Renames the 'selected' signal to 'changed' for better
consistency with other signals and to accurately reflect the signal's purpose.

UI/ui_tab_widget.hpp

@@ -24,7 +24,7 @@ class UI_tab_widget : public UI_item
     void from_json(const json &j) override;
     json to_json(size_t id) const override;
   signals:
-    void selected(size_t index);
+    void changed(size_t index);
 
   private:
     size_t m_selected_tab;