Replace wxSizer::RecalcSizes() with RepositionChildren()

The new method takes minimal size just computed by RecalcSizes() as its
argument making it unnecessary to store it as a member variable in the derived
classes such as wx{Box,FlexGrid}Sizer.

The old method can still be overridden for compatibility and by the derived
class that don't need minimal size when updating children.

include/wx/gbsizer.h

@@ -291,8 +291,8 @@ public:
 
 
     // These are what make the sizer do size calculations and layout
-    virtual void RecalcSizes() wxOVERRIDE;
     virtual wxSize CalcMin() wxOVERRIDE;
+    virtual void RepositionChildren(const wxSize& minSize) wxOVERRIDE;
 
 
     // Look at all items and see if any intersect (or would overlap) the given