PsyTimers, need to destroy their GSource when disposed or canceled.

[maartenuni]

In the unit tests we sometimes draw stuff without a GMainLoop running. this results in sources being invoked on disposed/finalized Timers..

[maartenuni]

They also need to dispose the GSource when, they are canceling a timer. E.g when setting the timer, we first cancel it. Then set a new time and register it with the thread. However, it can be the case that the thread has already initiated a new fire with a GSource and set a source_id on the timer. If it fires again, the timers->source_id is overwritten and we lose the reference to that source id and a newly created mainloop further in the tests is going to handle that source, on a timer that has been destroyed.