Use int instead of wxWindowID in wxNewId() and friends.

The functions are available in wxBase builds too, but wx/windowid.h
isn't. Rather than always including that header, just use int, for which
wxWindowID is a typedef. This keeps the functions available in wxBase
for compatibility and is consistent with how IDs are handled in other
parts of wxBase, particularly wxEvent.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74486 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775