Compile theme located src/main/resources with vaadin-maven-plugin for spring boot application

[vaadin-bot]

Originally by devnewton

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vaadin-maven-plugin expects themes to be located in src/main/webapp.

This is incompatible with spring boot jar packaged applications for which the src/main/webapp is ignored (cf http://docs.spring.io/spring-boot/docs/current/reference/html/boot-features-developing-web-applications.html).

Tricky maven manipulation could be possible (maybe https://vaadin.com/forum#!/thread/9721905 but not working for me with the 7.6.8 version), but they should at least be documented.

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Imported from https://dev.vaadin.com/ issue #20164