Fix confirm dialog buttons focus and tabbing
Fix confirm dialog buttons focus and tabbing

This reproduces the native confirm dialog behaviour:

- ‘confirm’ button focused by default
- shift+tab, tab to jump between confirm and dismiss buttons

This fixes mesosphere/marathon#2144

Raffo/marathon-ui,watonyweng/marathon-ui,janisz/marathon-ui,watonyweng/marathon-ui,mesosphere/marathon-ui,cribalik/marathon-ui,mesosphere/marathon-ui,Raffo/marathon-ui,cribalik/marathon-ui,janisz/marathon-ui