Turn nuxeo-platform-spreadsheet into a standalone "plugin"

Undo the merge of {{nuxeo-platform-spreadsheet }} into the main repository.

Merge nuxeo-platform-spreadsheet and marketplace-spreadsheet into a single repository bundling Java code, unit tests, marketplace package build and functional tests.

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