gtk-session: always sync modifiers for client events
gtk-session: always sync modifiers for client events

The channel state is not synchronous.

It may happen that we want to set and unset quickly a modifier, but the
guest modifier state hasn't been updated yet, and will still be seen as
unset, preventing the last unset change.

SPICE/spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,SPICE/spice-gtk,flexVDI/spice-gtk,fgouget/spice-gtk,Fantu/spice-gtk,Fantu/spice-gtk,guodong/spice-gtk,flexVDI/spice-gtk,elmarco/spice-gtk,Fantu/spice-gtk,guodong/spice-gtk,Fantu/spice-gtk,SPICE/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,fgouget/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,elmarco/spice-gtk,fgouget/spice-gtk,elmarco/spice-gtk,guodong/spice-gtk,elmarco/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk