Change behavior of reference field in editor

In TRANSLATE-3320 we implemented reference field mechanism. Reference field is a field which we compare tags with during editing (tag check, tag insert with shortcuts etc.)
By default reference field is "source text" for translate task and "target text at import time" for review task.

The problem:
We check if task is a review task by checking for "emptyTargets" task property. So if at list one segment in the whole task contains data in "target text at import time" that field is considered as reference. This causes that other segments with empty "target text at import time" fields are unable to handle any tags because they are compared to the empty field.

The solution
Need to change editor behavior to use field that contain data as reference. In case "target text at import time" contains data by default it should be used as a reference, if "target text at import time" is empty - then "source text" should be used as a reference.