Refactor the component layout

[javier-godoy]

Over the years, we have been stacking assumptions on how the component should look, while adding new features and migrating from Vaadin 8. In consequence:

• The component inherits from VerticalLayout
• There are three nested horizontal/vertical layouts (not even divs, but actual Flow components)
• It's not clear which part of the layout is public API (which reduces room for non-breaking refactoring)
• There is no clear API for styling

This change should be addressed in a major release.

[mlopezFC]

We discussed about this and the idea is to create a basic web component that will act as a layout to simplify the dom structure. It will not be published as an external web component at least initially but will be embedded in the same jar. So it will function 100% server side to maintain the API.