added wxDocument::AlreadySaved() and use it in OnUpdateFileSave() to ensure that...

added wxDocument::AlreadySaved() and use it in OnUpdateFileSave() to ensure that the "Save" menu item is enabled for new documents, even although they're not modified yet (otherwise the standard Ctrl-S key doesn't work in this case which is pretty annoying)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@56581 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775