Added new functions SetButtonMinSizeClass() and
SetButtonMaxSizeClass() that set a button's minimum
and maximum allowed size class during ribbon layout collapsing.
Size classes are values from wxRibbonButtonBarButtonState enum.
wxRibbonButtonBar::MakeLayouts() and
wxRibbonButtonBar::TryCollapseLayout() are modified to support
this new behaviour.
The modified wxRibbonButtonBar provate layout routines are
aware of wxRIBBON_BUTTONBAR_BUTTON_SMALL which is not implemented
yet in MSW wxRibbonMSWArtProvider::DrawButtonBarButtonForeground.
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