add extra timeout with higher priority to LOK flushing

add extra timeout with higher priority to LOK flushing

The normal idle has TaskPriority::POST_PAINT, which means that
if we get too busy, the idle won't be called for a long time,
meaning the queue will get longer and longer, making its processing
slower, and client interactivity will be very poor, with updates
possibly coming only when everything becomes idle.

The second timeout will flush the queue after a reasonable timeout.
I don't think there's an optimal value, so let's choose 100ms for now.

Change-Id: Ia1312a690aefd2c8628c82e0f42b2993802d8b1e
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/122728
Tested-by: Luboš Luňák <l.lunak@collabora.com>
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>