size of right/lower pane (positive arg sets the size of
left/upper one as before, 0 is the same as -1 before: choose
default)
2. OnSashPositionChange() virtual function added - it may return
FALSE to cancel position change
3. splitter sample and docs (couldn't compile them though) updated
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@901 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
