In _KeyNavContainer, only set tabindex="-1" on focusable widgets; fixes #4933.
In _KeyNavContainer, only set tabindex="-1" on focusable widgets; fixes #4933.

git-svn-id: 674a2d6b1e32e53f607eb824547723f32280967e@12318 560b804f-0ae3-0310-86f3-f6aa0a117693

harbalk/delite,kitsonk/delite,kitsonk/delite,harbalk/delite,Sam-Cummins/delite,Sam-Cummins/delite