Turn nuxeo-showcase-content-sample into a standalone "plugin"

Merge nuxeo-showcase-content-sample and nuxeo-marketplace-sample into a single repository bundling Java code, unit tests, marketplace package build and functional tests.

• Remove nuxeo-showcase-content-sample from addons/pom.xml.
• Create QA job https://qa.nuxeo.org/jenkins/job/master/job/plugins_nuxeo-showcase-content-sample-master/.
• Remove QA jobs addons_nuxeo-showcase-content-sample-master and addons_FT_nuxeo-showcase-content-sample-master.
• Update marketplace.ini.
• Remove all code in marketplace-showcase-content-sample and leave just a README explaining the deprecation.