Split the project into multiple modules

[pleku]

The parent module name is vaadin-button-flow-parent structure for submodules should be this:

    <modules>
        <module>vaadin-button-flow</module>
        <module>vaadin-button-flow-testbench</module>
        <module>vaadin-button-flow-integration-tests</module>
        <module>vaadin-button-flow-demo</module>
    </modules>

Couple notes:

• the component module should only have the unit tests there, not UI nor ITs
• the demo module can now start depending on other component versions, but should use the provided scope for those
• we bring in the component specific dependencies from flow-component-base, and only leave there the dependencies plugins that are required for all the components
  • this should be the first step of to get started with - remove dependencies for the component from flow-component-base, add those to the component instead.

This needs to be backported to the V12 version (1.2) and V10 & V11 versions (1.0).

The release builds will probably have to be reworked, the easiest approach is to create a new release configuration based on the old one and make it work.

[pleku]

There is a WIP branches for this:

• https://github.com/vaadin/flow-component-base/tree/modularization
• https://github.com/vaadin/vaadin-button-flow/tree/modularization

The changing of the version to 1.3.0-SNAPSHOT should be made in a separate commit, so that it is easier to

What is missing:

• make the version change in both repositories as a separate commit - so that it will not be cherry-picked together with the refactoring.
• remove all dependencyManagement parts from the component-base and declare the button specific dependencies versions in button's pom.xml
• make all plugin configuration to be managed, and declare those specifically in each component module that requires those (use profiles if necessary, like for OSGi)
• add vaadin-element-mixin as a direct dependency to flow-component-base as it is used by every in component (except iron-list)

There is a follow up issue for fixing all the other components