Check modifiers when handling TabSheet hot keys. Fixes #12178

Change-Id: I685dbf2c22e3b160632b811652bf3ab52d3ef6dc
Merge commit '478acb8e9aa7505cc6eacf219e87202191810a39'

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

Change-Id: If6a8663ad0c52789c1e01e750acbc343364d4767