KPluginMetaData: Use QFileInfo::completeBaseName for deriving plugin id
When we have plugin file names like "org.kde.plasma.icons", we would only get "org" as a plugin id, unless we specify an id explicitly in the metadata.
With the plans to use the KPackage metadata instead of the one from the applet binary plugins (see https://phabricator.kde.org/T14757 and https://phabricator.kde.org/T15094), we need plugin ids in the reverse domain notation to be derived from the file name.