wayland: Fix up key event translation

wayland: Fix up key event translation

The is_modifier field is supposed to be set if the key
would act as a modifier, not if any modifiers are currently
active. To fix this, introduce a private
_gdk_wayland_keymap_key_is_modifier function.

At the same time, make the hardware_keycode field in key
events actually contain the hardware keycode, not a copy
of the keyval.