Fix signed/unsigned comparison warnings in wxUniv wxNotebook.

Recent replacement of size_t wxNotebook::m_selection with int
wxBookCtrlBase::m_selection resulted in appearance of many warnings in wxUniv
wxNotebook. Fix them by removing some now unnecessary casts between int and
size_t and adjusting the remaining ones.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66092 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775