Enable copy & paste of internal tags

So far it is not possible to copy internal tags from the source of a segment to a target. Only copying of text is possible.

Copying of tags must be enabled through normal copy and paste. This only is true for the source of an opened segment. For not opened segments everything stays, as it is (tags can not be highlighted and copied).

• ensure, that no duplicate IDs are produced (= this cannot happen if already existing tags cannot be inserted; see below) (obsolete, inserting tags is not limited any longer)
• if a tag is inserted, that already exists, this tag is not inserted but all of the rest of the text and tags is inserted => done *
  * pls note: if a target contains the same internal tag more than once, it's only inserted once on opening the target (= the duplicated tags will not be inserted) = approved by Marc Mittag Oct 31; see also: _TRANSLATE-1440_
  removed completetely; the user shall do whatever they want to while editing, only the TagCheck on saving will point to unvalid internal tags (Marc Mittag 2018-11-05)
• if the corresponding closing tags are not copied and inserted, this is not validated or adjusted automatically. This is left for the final tag check on saving the segment. => new issue for Tag-Check (TL)