PR: Add range spinbox and range widget classes

[jnsebgosselin]

Add the RangeSpinBox and RangeWidget classes :

class RangeSpinBox(QDoubleSpinBox):
    """
    A spinbox that allow to enter values that are lower or higher than the
    minimum and maximum value of the spinbox.

    When editing is finished, the value is corrected to the maximum if the
    value entered was above the maximum and to the minimum if it was below
    the minimum.
    """

class RangeWidget(QObject):
    """
    A Qt object that link two double spinboxes that can be used to define
    the start and end value of a range.

    The RangeWidget does not come with a layout and both the spinbox_start
    and spinbox_end must be added to a layout independently.
    """