input-linux: add shift+shift as a grab toggle

We have ctrl-ctrl and alt-alt; why not shift-shift? That's my preferred
grab binding, personally.

Signed-off-by: Niklas Haas <git@haasn.xyz>
Message-id: 20190818105038.19520-1-qemu@haasn.xyz
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>