Calculates and returns the minimum possible size for the panel, considering the panel's user-min-width and user-min-height values as well as the x-margin-width and y-margin-height values.
Calls get-graphical-min-size to get the natural minimum size of the panel, and returns the minimum of these number and user-min-width and user-min-height, plus the x-margin-width and y-margin-height.