Don't send TabSheet focus and blur events to the server when moving the focus from one tab to another (#12971)

Change-Id: Ie496a403bdcfc833c4871efbae11453efa66c14e
Merge commit '401fa5c940d9b55a4f0b3f96db5990d57271fe82'

* commit '401fa5c940d9b55a4f0b3f96db5990d57271fe82':
  Don't send TabSheet focus and blur events to the server when moving the focus from one tab to another (#12971)

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

Change-Id: Ide3fc02d0beab8a0112a1e18ee417d0ec5097bb4