Improve use of `Enum` as widget choices and fix types

[qin-yu]

This PR originally attempts to fix the type warnings from #249. There are two modules using Enum for choices. I have enhanced both and resolved the type warnings along the way. The most significant changes include:

class RescaleType(Enum):
    NEAREST = (0, "Nearest")
    LINEAR = (1, "Linear")
    BILINEAR = (2, "Bilinear")

    def __init__(self, int_val, str_val):
        self.int_val = int_val
        self.str_val = str_val

    @classmethod
    def to_choices(cls):
        return [(mode.str_val, mode.int_val) for mode in cls]

class RescaleModes(Enum):
    FROM_FACTOR = "From factor"
    TO_LAYER_VOXEL_SIZE = "To layer voxel size"
    TO_LAYER_SHAPE = "To layer shape"
    TO_MODEL_VOXEL_SIZE = "To model voxel size"
    TO_VOXEL_SIZE = "To voxel size"
    SET_SHAPE = "To shape"
    SET_VOXEL_SIZE = "Set voxel size"

    @classmethod
    def to_choices(cls):
        return [(mode.value, mode) for mode in RescaleModes]

This approach provides an elegant solution for managing lists of string choices and their corresponding values.

[lorenzocerrone]

Looks good to me