Another regression from commit 74407aef. SystemWindow should
consume only Alt pressed/released events and let key input window
handle the rest of modkey events

Change-Id: I304701fe63ee86bad534ba003ea05991d06f433d
Reviewed-on: https://gerrit.libreoffice.org/21854Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>