Implement the "dummy" element

[potocpav]

Here's the PyImGui function: https://pyimgui.readthedocs.io/en/latest/reference/imgui.core.html#imgui.core.dummy

Implementation:

def dummy(width, height):
    """ Add a dummy element of a given `width` and `height`.

    Useful for custom-sized vertical or horizontal spacings.
    """
    return lift(imgui.dummy, width, height)

[potocpav]

Fixed in master.