FIX tag-handling in Transit Plugin

The Transit (file import) Plugin does not handle tags correctly anymore, it seems instead of (internal) tag-pairs two singular tags are generated, what leads to errors in tag-validations and other contexts. The parsing of the Transit files (editor_Plugins_Transit_File) has to be fixed and a test should be added to be able to test the plugin in the future (for testdata: see https://jira.translate5.net/browse/TS-2547)