Allow loading in Page and Group components

[ecarreras]

This pull request includes changes to refactor the Group and Page classes to extend from the Spinner class instead of ContainerWidget, and updates the corresponding tests to verify the new behavior. The most important changes include modifying the class inheritance and adding tests to validate the loading property.

Class inheritance changes:

• src/Group.ts: Changed the Group class to extend from Spinner instead of ContainerWidget.
• src/Page.ts: Changed the Page class to extend from Spinner instead of ContainerWidget.

Test updates:

• src/spec/Group.spec.ts: Added tests to verify the loading property behavior in the Group class.
• src/spec/Page.spec.ts: Added tests to verify the loading property behavior in the Page class.