when calculating layout and the return value from that will be used
for the minimum size. Added wx.FIXED_MINSIZE flag for when you would
like the old behavior but you should only need it when your desired
-size is smaller than the item's GetBestSize()
+size is smaller than the item's GetBestSize. When a window is added
+to a sizer it's initial size, if any, is set as the window's minimal
+size using SetSizeHints if there isn't already a minimal size.
2.5.1.5