custom posting of mouse,key events on main thread

This still bypasses vcl while keeping the processing of events on the
main thread which is what we want.

Change-Id: Ia7a6f5ef1ac546245715abe418d261b49df12d4c
Reviewed-on: https://gerrit.libreoffice.org/50274Reviewed-by: Aron Budea <aron.budea@collabora.com>
Tested-by: Aron Budea <aron.budea@collabora.com>