Adds some wxALIGN_* flags to increase ability to position item
within its allotted space.
Adds wxSHAPED flag that enforces proportional resizing on growable
items.
Adds a sample and updated documentation.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@4461 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
