1. Split{Horizontal|Vertical} now accept negative args to set the

1. Split{Horizontal|Vertical} now accept negative args to set the
   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