This class is used to describe widget position inside area
choosen by the container's layout.
x is in range 0.0 to 1.0 from left to right (0.5 is the middle)
y is in range 0.0 to 1.0 from top to bottom (0.5 is the middle)
Alignment is useful only if x_expand_allowed or y_expand_allowed
is True and the space given to the object is bigger than it's