Check and rework tag-only segment handling in import

The import of pure number tags has several potential side-effects that must be checked & resolved:
 
editor_Models_Import_FileParser_Xlf::extractSegment:

• copying source to target is dangerous in case of number-tags in source
• internalTag::hasText could be extended to return true when detecting number-tags. This would affect Bulk-Update, Pretrans & Pivot Pretrans and might make sense. -> check

Checks for empty segment content must respect number-tags: Segment is not empty then...