mi: Add a callback to notify driver about input event submission

With input threads there's no way to synchronize processing of events with any external source which makes it impossible to implement reliable integration tests that use input. Adding a hook at the end of mieqProcessInputEvents is enough for a custom driver to synchronize with the end of input event processing.

A driver using this API is here: p12tic/xf86-input-emulated!1

Refactoring of existing Xorg tests is here: xorg/test/xorg-integration-tests!1 (merged)

This work has already uncovered a serious regression in touch ownership input event handling.