WIP: re-factor line widgets

- make LineBaseWidget
- make LineWidget for plotting intensities under shapes layer
- make FeaturesLineWidget for plotting features from labels layer (grouped by a label column matching labels in layer)

src/napari_matplotlib/base.py

@@ -209,7 +209,9 @@ def current_z(self) -> int:
         """
         Current z-step of the napari viewer.
         """
-        return self.viewer.dims.current_step[0]
+        if self.viewer.dims.ndim < 3:
+            return slice(None)
+        return self.viewer.dims.current_step[-3]
 
     def _setup_callbacks(self) -> None:
         """