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').
@cahrens code in next line is appending " .icon-plus" with value of self.BOTTOM_AND_SECTION_BUTTON (not replacing it with " .icon-plus"). If you still want to refactor then we can do something like:
element_css = self.BOTTOM_ADD_SECTION_BUTTON + (" .icon-plus" if click_child_icon else "")