Many changes/fixes to wxFlexGridSizer implementation (no API changes):

- fix the problems addressed by the patch 1667343:
 * only distribute extra space between growable items, not all space
 * take hidden items and gaps into account for ALL grow mode layout
 * fix rounding errors by allocating the remaining pixels to the last item(s)
- refactor the code to avoid duplication between row/column cases
- use STL-like wxList methods instead of compatibility ones

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@45583 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775