Avoid duplicating timers in MessagePumpForUIQt

Since DoDelayedWork always returns the time of the next delayed work
to process, we would create a new timer for the same event every
time we process an event. This would cause a timer starvation on
Windows for example.

Instead of blindly creating timers, keep track of the requested delay
and create a new timer only if the new delay is shorted than the
currently scheduled one.

Change-Id: I2606231f0a48b8feb85f058a78eb772d26b222f3
Reviewed-by: Andras Becsi <andras.becsi@digia.com>