Implementations of wxPGProperty::ChildChanged() must now return changed value of...

Implementations of wxPGProperty::ChildChanged() must now return changed value of the whole property instead of writing it back to 'thisValue' argument. This change was done primarily for better compatibility with wxPython bindings, but should also be slightly more cleaner behavior API-wise. Breaks backwards compatibility, but not silently.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@60936 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775