Normally we handle Idle events directly without posting events
to the main event queue, as there seem to be no way to post them
to the end of the queue and this starves system events.

This prevents using this short-cut, as the default Windows event
processing doesn't know of this special handling.
Eventually this hack should be removed by simply always processing
all pending events...

This patch includes the follow-up commit 7b148b0e ("WIN move
native dialog hack into WinScheduler").

Change-Id: If9ae81ca7e847743f9251343e106dbf566371584
Reviewed-on: https://gerrit.libreoffice.org/65040
Reviewed-on: https://gerrit.libreoffice.org/65100
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>
(cherry picked from commit 5bb798a9)
(cherry picked from commit 7b148b0e)
Reviewed-on: https://gerrit.libreoffice.org/65065