Change return type of wxList::Member() to bool.

Change return type of wxList::Member() to bool.

It used to return a pointer in wxUSE_STL==0 build and an object in
wxUSE_STL==1 one making checking its return value difficult without provoking
warnings from either MSVC or g++ (see #11038).

Also, all the other occurrences of Member() already returned bool, including
the one in wxStringList so changing it to return bool in wxList itself is more
consistent.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61966 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775