TBX-Import: In TBX without termEntry-IDs terms get merged along autogenerated termEntry-ID

To reproduce:

• import a TBX in termCollection "test1", that has no termEntry-IDs
• import another different TBX in termCollection "test1", that has no termEntry-IDs
• Because of the merging of termEntries along the ID termEntries of both TBX are merged, because translate5 auto-generates IDs always starting with 1 at the beginning of the file.

Instead translate5 should auto-generate IDs that do not exist in translate5 database so far.