FocusTracker#isFocus gets out of sync when multiple blurs are followed by the focus

[oleq]

For the following structure registered in the FocusTracker (Parent and both Children):

Parent
   Child1 (focused)
   Child2

when the Child2 gets focused, blur events in DOM are fired both by the Child1 (which actually loses focus) and by the Parent (because FocusTracker adds listeners with useCapture).

ATM, only FocusTracker#_focus() clears the blur timeout, so in the situation above, only the second timeout is cleared and the first one is executed, which ends up with FocusTracker#isFocused sets to false despite the focus remaining within the FocusTracker's scope.

The solution is basically clearTimeout in FocusTracker#_blur.