Adapt LoadClassifierContainer to allow other file extensions

[fdsteffen]

When reloading a classifier magicgui's FileEdit widget currently filters for files with the extension *.clf.

# LoadClassifierContainer class from classifier_widget.py
def __init__(self, viewer: napari.viewer.Viewer):
    self._viewer = viewer
    self._clf_destination = FileEdit(mode="r", filter="*.clf")
    self._load_button = PushButton(label="Load Classifier")

Classifiers saved with another file extension (e.g. standard python .pkl files) can currently not be loaded again. Unfortunately, the FileEdit widget itself does not allow to deactivate the filter and choose another filetype. A possible workaround may be to include a filter RadioButtons widget to set the filter to all files *.

I could think of something along these lines:

def __init__(self, viewer: napari.viewer.Viewer):
    self._viewer = viewer
    self._clf_destination = FileEdit(mode="r", filter=None)
    self._filter = RadioButtons(
        value="*.clf",
        choices=["*.clf", "*.pkl", "*"],
        orientation="horizontal",
        label="Filter",
    )
    self._load_button = PushButton(label="Load Classifier")
    self._run_container = None
    super().__init__(
        widgets=[self._clf_destination, self._filter, self._load_button]
    )
    self._load_button.clicked.connect(self.load)
    self._filter.changed.connect(self.set_filter)

def set_filter(self):
    self._clf_destination.filter = self._filter.value