uix:GridLayout honour width/height for the col/row only if all the

widgets are setting size_hint_x/y to None. closes #954

To reproduce the issue this solves::

1) Run Kivy catalog, Choose the Button labeled 'GridLayout' from the left.

Notice that GridLayout takes up less area than is available to it if all the widget's in a row/column don't have size_hint_x/y set either None or between 0/1.

I feel the expected behaviour should be to honour width/height for the col/row only if all the widgets in that perticular row/col are setting size_hint_x/y to None and to ignore the width/height otherwise.