This updates the system loop integration to be on par with the
other backends. This includes:

1. Process LO user events before other LO events
2. Correctly handle elapsed QTimer events
3. Don't wait-yield in the main thread from a non-main thread

Change-Id: Ia17be032ae39dc4c7bfa44cadd22d85a1b9c4fbd
Reviewed-on: https://gerrit.libreoffice.org/43198Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>