Minor size optimization.
Factored out the tree inheritance related features from the Uize.Widget class and put this into a separate Uize.Oop.mTreeInheritance mixin module.
Promoted size to being a tree-inherited property of V2 widgets, with a CSS binding declared in the V2 mixin. This removes the responsibility for individual V2 widget classes to propagate the size down to child widgets, as numerous of them were doing.