Fixed TabSheet tab keyboard focus issues (#12343)

Change-Id: Ifacf4208e5124665115d13928463d87922282461
Merge commit '9ab700417ee5fe54329beee3130e6303f9420389' into master

* commit '9ab700417ee5fe54329beee3130e6303f9420389':
  Fixed TabSheet tab keyboard focus issues (#12343)

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

Change-Id: Id80bad07d004149c017ccee79994b4fbdfa71299