ShellTrayIcon: forward key presses too

StWidget::popup-menu is emitted when Menu/<Shift>F10 is pressed,
not released (for consistency with Gtk+), so we need to forward
that. Note that for key press we don't emit the matching key
release, because the app will take a grab and get the event directly
from X when the key is physicall released.

https://bugzilla.gnome.org/show_bug.cgi?id=721267