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.cpp

@@ -16,7 +16,7 @@ void UI_tab_widget::set_selected(size_t selected)
 {
     m_selected_tab = selected;
     emit ui_changed();
-    emit this->selected(m_selected_tab);
+    emit changed(m_selected_tab);
 }
 size_t UI_tab_widget::selected() const
 {
@@ -61,7 +61,7 @@ void UI_tab_widget::advance_tab()
         return;
     m_selected_tab = (m_selected_tab + 1) % m_tab_names.size();
     emit ui_changed();
-    emit this->selected(m_selected_tab);
+    emit changed(m_selected_tab);
 }
 
 void UI_tab_widget::from_json(const json &j)

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;