Build: remove <finalName> in maven-assembly-plugin goal

This will switch back to default ${project.build.finalName} and consider the
assembly result as resulting artifact of the build.
(cherry picked from commit 63a17ce1096e3c189f3ad2f838edca72c734cd34)
build: add a root pom

This root pom.xml has the main aggregator pom (build/org.eclipsescout.demo.parent) as maven module.
(cherry picked from commit 4898b5851c9acb8b2945401f7a73d329c3380bbf)