Fix ThreeComponentsSplitter focus cycle

[PureSin]

Allow tab to navigate focus between the 3 panels of ThreeComponentsSplitter by changing the traversal policy and removing ThreeComponentsSplitter as the focus cycle root. This allows for users of the component the control of the focus cycle instead.

Also allows for StripeButton to be focused.

@denis-fokin will use this PR to follow up on your comment: https://github.com/JetBrains/intellij-community/pull/760#issuecomment-387036701

[PureSin]

So you're ok with this change if I surround the new code with if (ScreenReader.isActive()) { ?

 if (ScreenReader.isActive()) {
  new code 
} else {
  old code
} 

?

[ignatov]

cc @denis-fokin