[DropDownMenu] Accept selectedMenuItemStyle prop

[bricka]

• PR has tests / docs demo, and is linted.
• Commit and PR titles begin with [ComponentName], and are in imperative form: "[Component] Fix leaky abstraction".
• Description explains the issue / use-case resolved, and auto-closes the related issue(s) (http://tr.im/vFqem).

This prop allows the selected menu item to be styled. We are achieving
this by passing it through to the underlying Menu component.

[DoubleU23]

? you pass the style object but don't handle/assign it. ?
but thanks for the test.spec

i think PR #5389 does the job

[oliviertassinari]

I'm closing this PR as have been quite inactive for a long time and because #5389 is promising.
Thanks for your time!