Support latest Vaadin Gradle projects

[tanbt]

Designer supports Vaadin 10 - 13 gradle projects.

Vaadin 14+ gradle projects should also be supported:

• Servlet (jetty) app: https://github.com/vaadin/base-starter-gradle
• Spring app: https://github.com/vaadin/base-starter-spring-gradle

Users should be able to create new Lit/P3 views, connect them to Java companion file and open the view when running the app.

Note: In Vaadin Maven projects, frontend dependencies are installed by Designer if they're missing. Gradle projects should also have the same support. Since this feature could be complicated, the implementer can split it to a new issue.

References:

• Following the latest platform release: https://github.com/vaadin/platform/releases/tag/20.0.0.alpha5
• Gradle snapshot version: http://tools.vaadin.com/nexus/content/repositories/vaadin-prereleases/com/vaadin/vaadin-gradle-plugin/20.0.gradle-SNAPSHOT/