Maven goal to create a widgetset

[vaadin-bot]

Originally by @hesara

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If a project does not already have a widgetset, it should be possible to create one with the Maven plug-in (e.g. "mvn vaadin:create-widgetset").

The location of the generated widgetset should be in the src/main/... directory by default but configurable with a parameter. The content of the created widgetset should be identical to that created by the Eclipse plug-in.

See also #9629.

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Imported from https://dev.vaadin.com/ issue #9918