Update root pom artifact id from "application-filemanager-pom" to "application-filemanager"

[slauriere]

By default the Maven release plugin uses the root parent name as tag, hence using "application-filemanager" as artifact id will make the release process simpler and will avoid ending up with release tags such as application-fliemanager-pom-2.5.5. As mentioned by @mflorea, the update is safe because com.xwiki.filemanager:application-filemanager was never used. It's not the case for the contrib version however: where the UI module has:

<xwiki.extension.features>
  tdelafosse:file-manager-application,
  org.xwiki.contrib:application-filemanager
</xwiki.extension.features>