get targetKey() {
      const parts = [self.regionId];
      if (isDefined(self.controlName)) {
        parts.push(self.controlName);
      }
      return parts.join("-");
    }
A key that should be unique in the context of the current annotation and current moment
based on the target of Anchor.
It allows distinguishing Anchors by their target (basically by area on the screen to which it was attached)
and groups Anchors with the same target.
Right now it is used to display only one comment per area on the screen.

@return {string} A key which points to a unique Anchor's target.