Made TabSheet tabs always become visible when focused (#12343)

Change-Id: I8f840bf4e45e257454d5e16c7b43f63858fd15d8
Merge commit '60aa66a'

Conflicts:
	client/src/com/vaadin/client/ui/VTabsheet.java

Change-Id: I7bda391a0e5678f85c04bd8ff9bfa65ba1c660b9
Merge "Merge commit '60aa66a'"