PositionerBox has bugs

Live Demo revealed a couple of bugs with the positioner box

• signal missing for toolbutton, probably due to refactoring. adding a test would be good.

Error Prompt:

Traceback (most recent call last):
  File "/Users/appel_c/work_psi_awi/bec_workspace/bec_widgets/bec_widgets/widgets/positioner_box/positioner_box.py", line 94, in _open_dialog_selection
    line_edit.textChanged.connect(self._positioner_changed)
                                  ^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'PositionerBox' object has no attribute '_positioner_changed'