Merge multiple attributes of the same type in TBX import

Attributes of the same type should be merged semikolon separated with a whitespace behind, if the same attribute occures multiple times in the same termentry on the same level (so those same on termEntry level should be merged, those same on languageLevel should be merged, and those on term-level. But if the same attribute occurs once on termlevel and once on languagelevel, it should not be merged).