Enhancement
The TabBar is to be rendered by an HTML template, a ThemeVar.
Code Migration
Almost all configurable css classes for the DefaultTabBarRenderer have been removed. It is still possible to create a new type of tabbar. The "tab-bar-class" is still available for this purpose.
Additionally, a new Java package named "layout.tabbar" has been created. All classes related to tabs were scattered in various packages in the project. All tab-related classes, except for TabComponent and the render classes, were migrated to the new package.
Test
Launch application and verify that the tabbar works exactly the same as it did before refactoring.