Use `magicgui` to generate GUI in `SliceWidget`

[dstansby]

When I implemented SliceWidget, I manually built the GUI and widgets using qtpy. It would make much nicer code if this was done with magicgui however. For an existing example of how to do this see:

napari-matplotlib/src/napari_matplotlib/scatter.py

Lines 130 to 136 in 081d4b6

	self._key_selection_widget = magicgui(
	self._set_axis_keys,
	x_axis_key={"choices": self._get_valid_axis_keys},
	y_axis_key={"choices": self._get_valid_axis_keys},
	call_button="plot",
	)
	self.layout().addWidget(self._key_selection_widget.native)

[dstansby]

Given the leaks I've fixed in #108 were caused by using magicgui, I'm going to close this as something that shouldn't be done.