core: Change MetaWaylandTextInput event forwarding to IMs

From the main commit:

We need to juggle with some things here to keep key event ordering
and accounting consistent.

The keyboard internal state changes (and maybe modifier event emission)
happening through meta_wayland_seat_update() should ideally happen
from the same key events that reach the client through wl_keyboard.key,
so that wl_keyboard.modifier events are emitted in the right relative
order to other key events.

In order to fix this, we need to decide at an earlier point whether
the event will get processed through IM (and maybe be reinjected),
thus ignored in wait of IM-postprocessed events.

This means we pay less attention to whether events are first-hand
hardware events for some things and go with the event that does
eventually reach to us (hardware or IM).

Closes: gnome-shell#5890 (closed)