This makes event handling easier where plugins queue other event frames
per frame. Our initialization guarantees that our evdev code is alway
the last plugin in the series so in the no-plugin case we just pass on
to that.
The effective event flow is now:
evdev.c -> plugin1 -> plugin2 -> evdev-plugin.c -> evdev.c
except that no plugins exist yet.
Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1217>