Make base widgets public available

[FloVanGH]

There are many request for better widget customization support in Slint. It is already possible to create custom widget libraries in Slint that are based on custom designs, two examples of that are SurrealismUI and coop. But for some use cases it could be easier to start with base widgets that provides a predefined behavior. We already have internal shared base widgets that a used by the different styles, that could be make public available.

Steps needed to make the base widgets public:

• API review of the base widgets and cleanup of the api
• Check if more base widgets are needed
• Find a way to make the base widgets available e.g. through import e.g. `import { BaseButton } from "base-widgets.slint";