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

include/wx/univ/notebook.h

@@ -201,10 +201,10 @@ protected:
     wxSize GetSizeForPage(const wxSize& size) const;
 
     // scroll the tabs so that the first page shown becomes the given one
-    void ScrollTo(int page);
+    void ScrollTo(size_t page);
 
     // scroll the tabs so that the first page shown becomes the given one
-    void ScrollLastTo(int page);
+    void ScrollLastTo(size_t page);
 
     // the pages titles
     wxArrayString m_titles;