scheduler: add method to get all schedulers for smartplugin

[onkelandy]

e.g. to easily remove all active scheduler on stop, etc. for scheduler in self.scheduler_get_all(): self.scheduler_remove(scheduler)

Feel free to improve/adapt the method, however I think it's convenient and useful when stopping/suspending plugins.

[Morg42]

Alternative method with smartplugin tracking its own schedulers...

https://github.com/Morg42/smarthome/commit/9974633b7a308762278c245406c67a1a366fdb90

[onkelandy]

Actually alternative is cleaner. Maybe we could add a remove_all method to make it even easier eg for deleting schedulers on suspend/stop..?