Expand prop_cpp_conditional support to sizers and containers

[KeyWorksRW]

Description:

The cpp_conditional property available for most forms couild be expanded to support containers and sizers. It could also support an else_conditional property. If a custom property dialog was created, we could offer suggestions such as specific platforms, specific versions of wxWidgets, _DEBUG or RELEASE, etc.

We could expand this to Python and Ruby as well, though is rather doubt it would be used much if at all (on the assumption that the level of complexity requiring condtionals beyond platform are less common thatn in C++ code).