Cleaned up the implementation of VTabsheet.selectPreviousTab()

Change-Id: I8e7fef3005173d7647b4dfda31f60978cbf5972a
Merge commit 'b7e184e30f8e5f1456344f3360c7e8e326c5ad2b' into master

Original commit message:
	Cleaned up the implementation of VTabsheet.selectPreviousTab()

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

Change-Id: I8798c2f8e5d5922aa4e7d1b9ce2617eaa1c4ab03
Merge "Merge commit 'b7e184e30f8e5f1456344f3360c7e8e326c5ad2b' into master"