Made calculating of DockMinSize for panel based on children DockMinSize

[scdmitryvodich]

This fix for calculating of DockMinSizes based on children minimal dimensions.
For cases when we have some complex structure, like this:

Group Panel (horizontal):
    Anchorable Panel # 1 (DockMinWidth = 150) "File Stats";
    Group Panel (vertical):
        Anchorable Panel # 2 (DockMinWidht = 150) "Tool1";
        Anchorable Panel # 3 (DockMinWidht = 150) "Color Picker";
    

When we decreasing width of horizontal Group Panel it's not take into account of DockMinWidth for panels # 2 and # 3. And these panels becomes partly hidden under Document panel (see attached screenshot)