split app.cpp into multiple files, add NodeScroll and use it to scroll the side panels, some fixes to the events system

engine/node_button_custom.cpp

@@ -46,11 +46,16 @@ kEventResult NodeButtonCustom::handle_event(Event* e)
         break;
     case kEventType::MouseDownL:
         m_color = color_down;
+        mouse_capture();
         break;
     case kEventType::MouseUpL:
         m_color = m_mouse_inside ? color_hover : color_normal;
         if (m_mouse_inside && on_click != nullptr)
             on_click(this);
+        mouse_release();
+        break;
+    case kEventType::MouseCancel:
+        mouse_release();
         break;
     default:
         return kEventResult::Available;