winrt: process all triggered timers in processEvents

If only one timer is processed in there it is possible that a reoccuring
timer which has a very low timeout blocks all the other timers from
being triggered. This high frequency timer might be the only one to
be triggered in every processEvents call.

Task-number: QTBUG-51888
Change-Id: I8a0026d1e8519171ab60d1b47c494a15d30328b3
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@theqtcompany.com>