input: store the grab serial of the keyboard
input: store the grab serial of the keyboard

The serial can be checked against the one passed to wl_shell.set_popup
or equivalent.
Reviewed-by: Daniel Stone <62e7e355d5d8400fa8384448796af0bf6be4ffae@collabora.com>

krezovic/weston,Tarnyko/weston-xdg_surface_present,xorgy/weston,Gnurou/weston,jonnylamb/weston,Gnurou/weston,jonnylamb/weston,udoprog/weston,sir-murray/weston,eyolfson/weston,udoprog/weston,kwm81/weston,kwm81/weston,Tarnyko/weston-xdg_surface_present,mchalupa/weston,krezovic/weston,giucam/weston,giucam/weston,Fantu/compositor-spice,xorgy/weston,Fantu/compositor-spice,eyolfson/weston,sir-murray/weston,mchalupa/weston,Fantu/compositor-spice