fix click event for actual target

TNL-138
@waheedahmed @symbolist @adampalay @cahrens 
Resolve the issue where when some users clicks on plus icon inside "button-new" ["New Section", "New Subsection", "New Unit"] then they see server error (KeyError: 'parent_locator').
Please refactor so it is clearer what this is doing--

something like element_css = self.BOTTOM_AND_SECTION_BUTTON if not click_child_icon else ".icon-plus"