Reference Vaadin-specific plugin when generating a project with Vaadin

The Vaadin Gradle Plugin is now available at https://github.com/vaadin/vaadin-gradle-plugin/ . The plugin will generate all files necessary for Vaadin to work properly, which will also fix spring-io/initializr#1122 .

When using Vaadin, could you please add the following line:

• id 'com.vaadin' version '0.14.3.7' to build.gradle
• id("com.vaadin") version "0.14.3.7" to build.gradle.kts

[mstahv]

🥳 Works like a charm! Tested both Maven and Gradle projects.

[mstahv]

If we find out time to incorporate our Gradle plugin build to our “platform build” in the next quarter, we can probably start doing the same as Spring Boot Gradle plugin does: define the library versions there and cut a new version of the plugin on every library release. Then we can get the rid of the separate Vaadin version declaration that we currently have for both Spring Initialzr projects and in the base-starter example in our GitHub. This is what we could remove after this change:

ext {
	set('vaadinVersion', "14.4.5")
}