This removes several attempts at reducing jitter in slideshow
animations. Now we have high-resolution (ie. not clamped to 10ms)
timers on Windows and a cleaner and simpler main-loop, we should
be able to use generic timer code-paths for all of this.

    This also allows us to further cleanup and simplify the main-loop
removing the now redundent post-yield handler concept. If there is a
short enough timeout, we will take just 1ms of delay before executing
a short timer anyway.

    Also removed some lingering comments from an old attempt to boost
priorities which broken audio playback.

    Tested: tdf#32861 - still works, audio still plays, no new jitter
            in animations that I tested.

Change-Id: Iadc5e2a48828a18a599a86a8df14cb2b75dd425e
Reviewed-on: https://gerrit.libreoffice.org/19947Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>