Add tags to target segment by highlighting word

It should be possible to add a tag to the currently edited target of a segment by double-clicking on the tag  in the corresponding source or "target at import-time" of the currently edited target.

• If a text string in the target is selected and the tag, that is clicked on, belongs to a tag pair, the tag pair should be added around the selected text string. Even if the same tag(s) are already present in the target tag (that may make sense and is up to the user and the tag check)
• If no text string is selected, the tag on that is double-clicked should be entered at the current position of the cursor in the target.

Please note: According to https://confluence.translate5.net/display/BUS/Editor+keyboard+shortcuts this functionality already exists via keyboard short-cut but not yet via click on the tag in the source or the target at import time. So only this needs to be added. Yet: According to tests this existing functionality does not correctly work anymore due to the quirks that we are about to repair with the new segment editor.