Fix [#37275] can't import 2.68a keymap

Now exported keymaps will still be usable accross versions, even if some operator properties disappear (write a warning in console in this case, instead of "crashing").

Also factorized a bit of code here!

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@61073 954f8c5b-7b00-dc11-b283-0030488c597c