avoid isValid wrong value in Form.Field is "required" is changed and value set after and no "changed" event raised

in ColorButton allow the color to fill avalailable space, correct support for "alpha", round the design

change how generateImage is done in drag to avoid double some child elements. change how padding is handle in ContentEditable to keep it when cloned

in TextArea and Entry correctly handle "placeholder" attribute and in drag keep the "value" in the generated image