napari widget Image shift controls does not work properly

[ondrejstranik]

Bug description
error message issued when arrows buttons in the napari widget are pressed:

ERROR [ExceptionHandler] Traceback (most recent call last):
  File "C:\Users\ostranik\Documents\GitHub\ImSwitch\imswitch\imcommon\view\guitools\naparitools.py", line 265, in _on_up
    self._do_shift(0, -self._get_shift_distance())
  File "C:\Users\ostranik\Documents\GitHub\ImSwitch\imswitch\imcommon\view\guitools\naparitools.py", line 281, in _do_shift
    for layer in self.viewer.layers.selected:
AttributeError: 'LayerList' object has no attribute 'selected'

Correction

change in the source naparitools.py methods on_reset(self) and _do_shift(self):

self.viewer.layers.selected --> self.viewer.layers.selection

[kasasxav]

@ondrejstranik thank you! Would you want to open a PR with the changes?