based on code from Niki Spahiev and lets you specify a row and column
for each item, as well as optional column or row spanning. Cells with
not item assigned to it are just left blank. Stretchable rows or
columns are specified and work the same as in wxFlexGridSizer.
A few other minor changes too.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@14431 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
