Updates DesiredSize of the DockPanel. Called by parent Control. This is the first pass of layout.
protected override Size MeasureOverride(Size constraint)
Children are measured based on their sizing properties and
Each child is allowed to consume all of the space on the side on which it is docked; Left/Right docked
children are granted all vertical space for their entire width, and Top/Bottom docked children are
granted all horizontal space for their entire height.
|Constraint size is an "upper limit" that the return value should not exceed.
|The Panel's desired size.