Make wxNewId() and others return/take wxWindowID rather than int

wxWindowID is a typedef of int, so nothing should really change, except
for the improved readability.

Closes https://github.com/wxWidgets/wxWidgets/pull/1682

interface/wx/utils.h

@@ -484,7 +484,7 @@ int wxFindMenuItemId(wxFrame* frame, const wxString& menuString,
 
     @header{wx/utils.h}
 */
-int wxNewId();
+wxWindowID wxNewId();
 
 /**
     Ensures that Ids subsequently generated by wxNewId() do not clash with the
@@ -492,7 +492,7 @@ int wxNewId();
 
     @header{wx/utils.h}
 */
-void wxRegisterId(int id);
+void wxRegisterId(wxWindowID id);
 
 /**
     Opens the @a document in the application associated with the files of this